Perhaps the thief class is in need of changing to promote an atmosphere that doesn't end in log out to avoid fighting the thief? I have encountered several thieves, every one has attacked me while I was fighting, or attacked others while they where fighting, and never left the city. If you do not die near instantly from their damage, or them picking you off while weak, it is simply them hiding. Perhaps it is something to look into because this just pushes an environment with less players. 5 players on, thief will harass any of them, 2-3 will log off. while intrinsically thieves may seem fine, but I do not think a games health is better because people log off from a fight
Thieves?
Alright, I'll bite.
If you poisoned yourself(multiple ways, easiest probably purchasing herbal brews from the tower of magic) before going into town while knowing there is a thief online who previously failed a blackjack against you, you'd find them almost a non-issue.
But regardless of that, rangers/druids/ninjas have the same mechanic of being able to hide so you'd have to tar them all with the same brush.
My experiences fighting the other classes is limited but there seems to be a general opinion ninjas are also in need of reworking because of similar problems. People will just leave.
Rangers ambushing you as you recall is a much less frequent occurance. The thieves I have fought prey on people as their attention is on someone else. I have had to stop a thief from killing someone who just killed me because the character in question was very hurt. Those actions do create an environment where people log off instead of risking a stupid loss.
If said thief is uncaballed and not a Qrace, there's really no wrong-doing here. Is it on the trashy side? Definitely. Is it legit? Unfortunately, yes. However, there's a good solution just one north and one east from Central Sq in Val Miran (IIRC) 