Kaidan's romance pretty much dies the instant you choose the second Renegade option during the conversation where you find the huskified Cerberus trooper on Mars, regardless of what other dialogue options you've chosen up to that point, or which dialogue options you choose later. At least, that's the case if you imported a romance from ME1. I don't believe anything short of a full Renegade approach on Mars will kill Ashley's romance, though.
If you mean after they've been locked-in as Shepard's romance, then no. You have to either reject them during the locking-in conversation (the one at Apollo's Cafe in Kaidan/Ashley's case) or lock-in someone else's romance beforehand. Terminator-HIX