Generally, if your astropathic signals leak, it means that your bridge crew is in the middle of a mutiny.<br/><br/>Technically, the Eldar can also intercept and decode astropathic signals, but they probably don't need to.  The Dark Eldar don't have any ability to and couldn't care about astropathic signals in the slightest, though if they really needed one, they would kidpap and astropath an torture him or her until they get the information they want.<br/><br/>The Orks can't intercept astropathic signals, though a sufficiently flash wierdboy could probably overload the local psychic landscape with WAAAGH energy to jam astropathic signals or even kill any astropaths receiving the WAAAAAGH directly into their minds.<br/><br/>The Tau have absolutely no ability to meddle with astropaths, and believe their abilities to be a matter of quantum entanglement.  Naturally, every single Tau experiment to try and understand the nature and capabilities of astropaths has failed, and they generally ignore them.