Hi Neetu,
               I have wrote the trigger and it is preventing the new task insertion if the no longer with the firm check box is checked in contact .
               Now the trigger is working ,but when we click on the save button , it is showing the error which i have written in the trigger,Can we                        redirect from the task page to contact record page by writing any logic in the trigger??
             
                 My trigger is 

                     Trigger TaskInsertPrevention on Task(before insert)
         { 
                  Set<Id> taskSetId = new Set<Id>(); 
                  for(Task task :Trigger.New)
                     {
                          String s1 =task.WhoId;
                          if(s1.startsWith('003e'))
                              {
                                taskSetId.add(task.WhoId);
                               }
          
                   for(Contact  con :[Select Id , Contact.No_longer_with_firm__c  from  Contact where Id In : taskSetID])
                         {
                    if(con.No_longer_with_firm__c == True)
                         {
                        task.addError('You cannot create a new task if the contact is no longer with the firm');
                         }
                          }
                      }
          }