I've been the unintended concentration-breaker in that situation. In my experience it's not about "seniority privilege"; it's that people are really bad sometimes about noticing these things. You intend to say "hello" in passing, the other person asks about that bug you filed, you say something, and before you know it you're having a full-on design discussion just outside somebody's cube wall. That's no good.
You really have two goals: make this conversation stop, and help prevent the next one. The same remedy addresses both: politely point out the problem and ask them to move. It'll take several times (with different people) and some people will need repetition, but if your set of coworkers is reasonably stable, you can change the culture over time.
Don't use a sign. During a conversation, walk up to the group and say something like "I'm sorry to interrupt, but could you please move this conversation? I'm having trouble concentrating". After you do this several times you should start seeing the participants initiate the move ("hey, let's move this to a conference room so we don't bother Dave"). In the meantime, remain polite and professional but don't be afraid of speaking up.
This won't work if your workplace has a lot of turnover (so it's never the same people) or if you work with rude people who revel in this sort of thing, but for most of us it's the place to start, and it may be easier than moving desks.