I vote for 'be honest', but that's just how I feel I think I would like to be treated.

Pretty sure s4ndm4n2006 referred to the general case of people not being honest about it, not you personally.

I would just say how you feel. If you don't feel romantic attraction, tell him.

In fact, you don't tell us why you don't consider him as a romantic interest. You just say he is some guy you hadn't noticed, right?

So if you don't feel like going out of your way to explore if there is a possibility of a romantic connection with a collegue, just say that.

Of course I don't mean for you to tell him he is too short and not attractive enough, if that is what you think.
But when you tell him you won't date him because he is a collegue, what do you do if he quits his job, then asks you again? I mean, I am sure it happened once.
Same with saying, lying, you have a boyfriend, or aren't looking for a relationship.

Do people here really think that a person hearing an excuse will think, "Oh, she meant to say that she doesn't think I meet her mating requirements. But she was so nice about telling me. She is so considerate in rejecting me. I feel better about myself now."?

These white lies are toxic, I feel.