If you simply want to restrict the Assigned to field so that it can never be the same as the Caller, update your Reference qualifier on the Dictionary Override for this table (incident or whatever). If you have something existing like
roles=itil... put that in quotes and add ^sys_id!=' + current.caller_id with a javascript prefix, so it looks something like this
If you only want to restrict this when the Assignment group is a certain value, you can try to add this in an if condition in this string, or call a Script Include to do the filtering if it gets too complicated.
