Surfacers in the Underdark, take 3. (Extremely easy fix)

[Cloak and Lager]

I noticed that there have already been two suggestions on this topic, but they were rejected due to difficulty of implementation.
Whilst server-wide interaction should be encouraged, and Andunor does this perfectly; there has been an increasing trend of large Surface parties going down to the UD to grind dungeons all day - repeatedly kicking out and killing UD characters in the process without impunity due to the much smaller playerbase of the UD server.
The solution to this is not to discourage the presence of Surface characters, as it can further roleplay and story development. In all the cases I've mentioned, the underlying reason for the presence of the Surfacers was either "boredom with Surface dungeons" or a perceived higher XP gain.

This problem would be solved if all Surface characters got a reduced xp gain from UD dungeons. It would still allow presence for roleplay reasons, but there would no longer be an incentive for Surface characters to make the UD their primary adventure server.