Try the truth. "I am not in the least bit interested in you in that way." If that is not enough, start ignoring them whenever the interaction gets to that point. Nothing stops a boner like hitting a stone wall, except when it belongs to a rapist, and for this reason I ask you to take caution. Interact with guys normally, but once the conversation starts demanding more, literally ignore them. The whole I'm taken and I'm lesbian thing is old plus there are too many people that don't care anyway. You tell a guy you're a lesbian and you might just get him more interested.
Just act psycho and he'll leave you alone for sure. Start talking about how you can't wait to meet his parents, act INSANELY jealous and emotional over literally nothing, and talk non stop about babies. This might only work on a guy you just met, though, not one that knows you and is genuinely interested. The best response would probably be the blunt, "I'm not interested in you."
Yeah, there as a blind guy talking to me and I was bored and wanted it to stop so I told him he was clear to cross the street. It cost me the dry cleaner to get the blood off, but it was totally worth it.
I think everyone has made some excuses to other people at least once in their lives. Sometimes I do it when my friends invite me to go out and I'm not in the mood/tired. But other times I just tell them the truth and they understand #introvertedpeopleproblems xD