Most services packages will ignore a /kill for a Service.
Like Dukat said, if you don't trust them, don't let them have an O:line. Even if you trust the person, make sure they understand the rules, and simply take their O:line if they abuse it by trying to kill other opers or services.
The simplest way to do that:
- Comment the O:line in the conf.
- /kill the abusive oper.
First of all, services can't ignore a kill, they have to reintroduce the killed user, because as each server passes on the kill, it removes the service client.
If usermode +S on a service isn't working, it's because your U:Lines aren't configured properly. Fix them. IRCops should never have +S because that is strictly for services.