Anyway to keep allies from roaming around your systems?

[MegaTrain]

I am in the middle of a PBW game, and a few minor skirmishes have broken out, but currently I am (Trade & Research) allies with all the other human players.

However, one guy (YOU KNOW WHO YOU ARE), keeps sending these annoying little uncloaked escort vessels throught my populated systems just to "look around" (I don't believe he is laying mines--the ships are too small). Is there any way, short of breaking our treaty, to keep him from doing that? Can I attack an individual ship without declaring war on the entire empire? I guess I could remove a couple of warp points, but that might inconvenience me as well.

------------------
-MegaTrain-
Athlon 1.3 GHz running at 1.6 GHz on an iWill KK266