You cannot stop force of nature; if you stimulate someone enough they will orgasm if no spell blocks the function. So only way avoid it is to limit your own modifier in charisma check and you can do indeed give yourself penalties to your own charisma check (to point whatever you do always renders subject sickened for 5 minutes). Another way is of course drive someone close to orgasm and then stop but nat 20 can occur which causes person instant orgasm if you do not have -6 or so penalty to own check (assuming +0 charisma mod).