services can't prevent people from sending private messages, so you are looking at the wrong program. Only the ircd can prevent people from doing things. So you would have to change the ircd to only allow people who have usermode +r to send pms.
There is a usermode that prevents unregistered ppl from pm'ing you (+R - depending on your ircd) but they can still pm any1 else and +R can be unset as well. Maybe set +R on connect on all you clients?