You just Gotta say look I've told you before and I don't want to break your heart but you are insisting. I'm gonna ask you to stop talking to me now. You've asked me out I've said no.
Trust me, it's kind of the guys fault but unfortunately it's sort of your fault for not being mean. Yes I meant that. You have to very brutally honest otherwise guys might think you're playing hard to get
Hopefully I don't "jump around" too much with this.
To me you were honest. Who knows what he is thinking. He wants to be "friends", fine. Is he able to separate out being friends vs. being attracted to you? Who's to say... The thing is, you don't know what his ulterior motive is of this "friendship." So far it seems that he's just asking stuff you would want to know be it if the person was the interested in you or not.
The thing is, you'll want to sort out your thoughts about this guy first. From what I have read from the comments, you're a bit "cautious" with this guy since he got your number from who knows where. You've already stated that you're "kinda seeing someone." Who's to say he wants to be "waiting in the wings." That's his choice.
Things get extra sticky since he knows you from work. Just talk to him in person that you're not interested. There's such a thing as "you have to be cruel to be kind" (I think I remember that from a song). Seems to me this guy either isn't getting it or doesn't want to get it (the whole, no means "not now" sort of thing).
Just tell him you are talking with someone else then continue ignoring him. If he doesn't leave you alone then just threaten to tell the managers at your job. You need to be firm and follow through with this if he doesn't leave you alone.
He isn't getting that you are not available. Just be firm with him and tell him to stop insisting you aren't interested. If he is a jerk and insist ignore him block him. at work only talk to him about work related matter nothing more. other than that continue as normal business.
"Hey, it's really sweet that you like me and text me but like I said, I'm seeing someone and I really don't want it to be awkward when he and I are hanging out and I have to explain your texts. I hope that makes sense to you. See you at work! "