I seriously hope no non-werewolf is spanning with the wolves against his own good. That would seriously break the game. I have no intention that that's happening, though.
There are subtle ways, of course. When the alchemist of last game was persuaded into killing Trieste, he wasn't aware it was done by an actual werewolf. He was merely tricked.
If someone falls in love with the werewolves, he doesn't become an ally of the werewolves. Instead, the two of them need to survive against all other players, including the other werewolves. They best keep that hidden, else the other werewolves would not want such a leak in their numbers (It's pretty common in games with one werewolf in love with a peasant that when the werewolves are apparently winning the game, the couple passes some werewolf names over to the peasants to even out the numbers).
The executioner can only use his/her special ability while the lynch votes are going on. The execution is theoretically instantly and the target dies without any chance of a save, even from the priest. The regular lynch continues afterward, but the vote from the lynched person of course no longer counts.
The executioner doesn't need to have any votes on him or her, neither does his or her target. He or she ca just pick someone that dies. Of course, if there are a lot of votes against the executioner, it makes sense for him or her to kill someone else in order to prove his or her own innocence.