Blocking NET SEND is easy. Use your GPO to disable the Alerter service. That will stop the machine from being able to send or receive NET SEND messages.
You can’t block xls and doc files as these aren’t programs. They are files which hold information. XLS files are Excel spread sheets and DOC files are Word documents. You can use a GPO to block Excel and Word from running, however this will probably create a problem as most businesses use Word and Excel often.