diff --git a/app/controllers/user_admin_controller.rb b/app/controllers/user_admin_controller.rb
--- a/app/controllers/user_admin_controller.rb
+++ b/app/controllers/user_admin_controller.rb
@@ -184,7 +184,7 @@
 
     operation = params[:operation]
 
-    if operation!='add' and operation!='remove'
+    if not ['add','remove','assign'].include? operation
       flash[:notice] = 'You did not choose the operation to perform.'
       redirect_to :action => 'contest_management' and return
     end
@@ -203,8 +203,10 @@
       if user
         if operation=='add'
           user.contests << contest
+        elsif operation=='remove'
+          user.contests.delete(contest)
         else
-          user.contests.delete(contest)
+          user.contests = [contest]
         end
         note << user.login
       end