I just had to cold-turkey shut down Ashley with the force of a thousand suns, after Virmire. Idiot stealth-romanced me again, as usual. The only way to avoid it is to literally skip out on a conversation, when you have to say "You're out of line!" when she brings up letting aliens onto the Normandy. Don't do that and turned down Liara? Guess what, it's time for your Shepard to be completely rail-roaded into romancing her whether you like it or not. Like she asks "so you still want me, baggage and all", it's like... What? I don't 'want' you like that Ashley, and it makes you look really freaking creepy how you thinking you're hot and you think expressing signals to him automatically makes him want you. Where was my "I don't want you." dialogue option? No where to be seen, Shepard HAS to want her if he didn't want Liara. You don't get a single dialogue option before post-Virmire to end it, and until then you're stuck with your Shepard basically auto dialoging his romantic feelings to someone who doesn't even remotely match his personality or your roleplay of him. It's infuriating.
So I had to be a complete dick head to her at the locker scene, which is how you can break it off. But honestly, it severely annoys me that there was even anything TO break off. As far as I'm concerned, he doesn't want her, never wanted her, has no interest in dating her, and would turn her down instantly if he asked. But I don't have that choice. Funny how people complain about auto dialogue in ME3 when ME1 is far worse about it. Yeah Shepard says some dumb auto things in ME3, but nothing as core to roleplay and character as forcing you into romance.
Speaking of forcing you into a romance, Liara does also suffer from this (ME1: "I sense a connection between us" - you have to shut her down hard, otherwise the game thinks that you're in a relationship).
ME3 - The game starts thinking you're in a relationship with Liara, if you start making paragon conversation with her on the Citadel (unless you tell Liara to work herself to death, Aethya starts sprouting about you and Liara are lovers). Liara, in ME3 stars to come off as creepily clingy, even if you're not in a romance with her (her final scene really spotlights).
That being said, I can sort of get Liara's crush on Shepard - for all intents and purposes we're the closest thing she's scene to the conquering hero. I just to like having the writers try and cram her down my throat.
An additional problem is that by ME3, she's become more of a plot device more than anything else. Compared to other characters, she has no real flaws, which kinda hints that she's become MW's waifu.