Do Not Trust Vega_of_alandria

I notice you didn't really explain the circumstances of what you say he did. Do you mean he stole from a trade or raided stuff you left unprotected.
 
If you left chests unlocked, that anyone has access to, you can't blame them. It's a faction server, meaning raiding, stealing items. If anyhow this person was able to steal valuables due to be given access in that area, you can't control what they're going to do with that access. Just on a final note know that you cannot trust anyone and it's a factions server, anyone can backstab you any moment in time.