Here's my suggestion: force cross-dc travelers to select a category, thus barring them from posting anything in the "other" tab.
For example, if I travel from primal to aether, I would need to select "dungeon" or "trial" or "raid" or "high end duty" or "hunt" or "fate" or whatever else, but I would not be able to make a party finder in the "other" tab.
The reason: There are too many ads for clubs under the "other" tab. Imagine, it's a slow night, maybe you're looking for one of those omnicrafters to make you something, maybe you're looking to trade something, maybe you're looking for a gposer because you don't have the patience for it yourself, or maybe you're just looking for a place to kill some time at. You open party finder. There are 68 listings on the "other" tab. You start looking through them. There are 8 ads for the same place, you can't scroll without seeing them. Then you see at least 4 other places with at least 3 ads up themselves, and pretty soon you realize that things aren't as they first appear. You keep seeing the same things over and over again, so much so that it's difficult to find anything else except for that one club that's really overzealous with their advertising.
So how does limiting traveler pf capabilities help? I start sending in reports for places that have too many ads. This, of course, means that I need to collect names and worlds. In the course of doing this, I've discovered that very few people with these ads up actually reside on the data center they're advertising on. In fact, the majority of them come from aether, closely followed by dynamis.
Why not just blacklist them to not have to deal with them? The reason's pretty simple. As of this moment, even with the enhancements coming with the blacklist system in dawntrail, we're still going to be capped at 200 users. With how often these places pop up and how overzealous most of them get with their advertising, you'd be hitting that cap pretty fast if you just blacklisted each and every one of them.
This solution would not be a magical cure for this problem. It's incredibly easy to just create a character on the same data center you want to advertise for. Same issue, different names. However, it would, at the bare minimum, act as a deterrent.