diff --git a/lib/Synergy/Reactor/Rototron.pm b/lib/Synergy/Reactor/Rototron.pm
index fc03767b..e0dd356e 100644
--- a/lib/Synergy/Reactor/Rototron.pm
+++ b/lib/Synergy/Reactor/Rototron.pm
@@ -116,19 +116,21 @@ after register_with_hub => sub ($self, @) {
 };
 
 sub handle_manual_assignment ($self, $event) {
-  my ($username, $rotor_name, $from, $to);
+  my ($username, $rotor_name, $from, $to, $force);
 
   my $ymd_re = qr{ [0-9]{4} - [0-9]{2} - [0-9]{2} }x;
-  if ($event->text =~ /^assign rotor (\S+) to (\S+) on ($ymd_re)\z/) {
+  if ($event->text =~ /^assign rotor (\S+) to (\S+) on ($ymd_re) (\/f(?:orce))?\z/) {
     $rotor_name = $1;
     $username   = $2;
     $from       = parse_date_for_user($3, $event->from_user);
     $to         = parse_date_for_user($3, $event->from_user);
-  } elsif ($event->text =~ /^assign rotor (\S+) to (\S+) from ($ymd_re) to ($ymd_re)\z/) {
+    $force      = $4;
+  } elsif ($event->text =~ /^assign rotor (\S+) to (\S+) from ($ymd_re) to ($ymd_re) (\/f(?:orce))?\z/) {
     $rotor_name = $1;
     $username   = $2;
     $from       = parse_date_for_user($3, $event->from_user);
     $to         = parse_date_for_user($4, $event->from_user);
+    $force      = $5;
   } else {
     return;
   }
@@ -161,6 +163,39 @@ sub handle_manual_assignment ($self, $event) {
     $assign_to = $target->username;
   }
 
+  my (@okay, @errors);
+
+  for my $date (@dates) {
+    if ($force) {
+      # Remove any 'unavailable on' overrides
+      $self->availability_checker->set_user_available_on(
+        $assign_to,
+        $date,
+      );
+    } else {
+      my $debug = [];
+      if ($self->availability_checker->user_is_available_on($assign_to, $date, $debug)) {
+        push @okay, $date;
+      } else {
+        push @errors, @$debug;
+      }
+    }
+  }
+
+  @dates = @okay;
+
+  unless (@dates) {
+    $event->reply("$username was not available for any of those dates:\n" . join("\n", @errors));
+    $event->reply("retry with /force at the end to override");
+
+    return;
+  }
+
+  if (@errors) {
+    $event->reply("$username was not available for these dates:\n" . join("\n", @errors));
+    $event->reply("retry with /force at the end to override");
+  }
+
   $self->availability_checker->update_manual_assignments({
     $rotor_name => { map {; $_->ymd => $assign_to } @dates },
   });
diff --git a/lib/Synergy/Rototron.pm b/lib/Synergy/Rototron.pm
index 385a6ee2..bc1c8ca8 100644
--- a/lib/Synergy/Rototron.pm
+++ b/lib/Synergy/Rototron.pm
@@ -457,14 +457,24 @@ package Synergy::Rototron::Rotor {
 
     if (my $username = $self->manual_assignment_for($rotor, $day)) {
       my ($user) = grep {; $_->{username} eq $username } $self->_full_staff->@*;
-      return $user if $user;
-
-      $Logger->log([
-        "no user named %s, but that name is assigned for %s on %s",
-        $username,
-        $rotor->name,
-        $day->ymd,
-      ]);
+
+      if ($user && $self->user_is_blocked_on($user->{username}, $day)) {
+        $Logger->log([
+          "user %s manually assigned but they are marked unavailable for %s on %s, skipping",
+          $user->{username},
+          $rotor->name,
+          $day->ymd,
+        ]);
+      } elsif ($user) {
+        return $user;
+      } else {
+        $Logger->log([
+          "no user named %s, but that name is assigned for %s on %s",
+          $username,
+          $rotor->name,
+          $day->ymd,
+        ]);
+      }
     }
 
     my $weekn = _week_of_date($day);
@@ -665,16 +675,42 @@ package Synergy::Rototron::AvailabilityChecker {
     return;
   }
 
-  sub user_is_available_on ($self, $username, $dt) {
+  sub user_is_available_on ($self, $username, $dt, $debug = []) {
     my $ymd = $dt->ymd;
 
+    unless ($self->user_typically_works_on($username, $dt)) {
+      push @$debug, "$username does not work on " . $dt->ymd;
+      return 0;
+    }
+
+    if ($self->user_has_leave_on($username, $dt)) {
+      push @$debug, "$username has leave scheduled on " . $dt->ymd;
+      return 0;
+    }
+
+    if ($self->user_is_blocked_on($username, $dt)) {
+      push @$debug, "$username is marked unavailable on " . $dt->ymd;
+      return 0;
+    }
+
+    return 1;
+  }
+
+  sub user_typically_works_on ($self, $username, $dt) {
     if (my $user = $self->user_directory->user_named($username)) {
-      return 0 unless $user->hours_for_dow($dt->day_of_week);
+      return 1 if $user->hours_for_dow($dt->day_of_week);
     }
 
+    return 0;
+  }
+
+  sub user_has_leave_on ($self, $username, $dt) {
     my $leave = $self->_leave_days;
-    return 0 if $leave->{$username}{$dt->ymd};
 
+    return 1 if $leave->{$username}{$dt->ymd};
+  }
+
+  sub user_is_blocked_on ($self, $username, $dt) {
     my ($count) = $self->_dbh->selectrow_array(
       q{SELECT COUNT(*) FROM blocked_days WHERE username = ? AND date = ?},
       undef,
@@ -682,7 +718,7 @@ package Synergy::Rototron::AvailabilityChecker {
       $dt->ymd,
     );
 
-    return $count == 0;
+    return $count ? 1 : 0;
   }
 
   sub set_user_unavailable_on ($self, $username, $dt, $reason = undef) {