How to handle flirting at work?

Ok, so there is a guy at work that flirts with a lot. While I used to not mind it because he's super cute! He just recently told me he has a girlfriend. I DO NOT want to get involved with that and I need to know how to keep it professional. But this is going to be hard to do because I have already flirted back because I thought we were both single at the time. So what would make this transition easier?
How to handle flirting at work?
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