How do you punish liars?

Make them bite on a bar of soap for a long time. Always works.

Muse is a good band. Perhaps you know of them? If someones a liar deal with it, it's who they are. This is probably morally wrong but I always feel good when I catch the person red-handed in lying. Funny how they turn all red and embarrassed. If you fall for their tricks it's your fault, not theirs, you shouldn't have listened to them. If you get really mad you could just... slap them or never talk to them again.

Being who they are, they most likely have treated others the same way and most often than not, you'd also get to know those that they have tricked.

If ever I suspect them for being like that, I would conspire (either with other individuals that were tricked) something really intricate which would eventually lead to cornering that creature into admitting their faults.. in front of plenty of many..

The more victims the creature was able to trick, the more unlikely their reasoning to excuse themselves out of it would be believable..