You can only post to clarification for payment lot processing, e.g. FP05 and FP25. FPE1 should not be used for a payment posting. FPCJ cannot post to clarification either. For FPCJ you can issue a message which prevents the user from posting or force them to post on account.
You could assign a clearing category which is only used for collective accounts and which allocates to clearing variant which checks for total balance...