Hush Parasite! You are giving away one of my little tricks.
More specifically, you should lay as many mines as possible on the wormholes, to take down sneaky vessels without enough minesweeping support (I will usually send a minesweeper if I am attempting to sneak in a hostile nebulae).
Of course, it will only work if you lack any treaty with the crafty Empire, and is of no use when one of your allies is busy planting a fleet in that annoying nebulae right in the middle of your Empire.