Is there a way to let guys know early on that I [24F] am not romantically interested before having to outright reject them so that we can both save face?

So this sounds a little vain, but for context I'm a young woman who'd probably be classified as conventionally attractive. I'm also pretty good at reading signals from guys to tell if they're interested or not, so I'm wondering if there's something I'm missing (through words, actions, body language, etc.) about letting men know early on that I'm not interested? This has come up in my life before, but I was thinking about it because of a recent situation.

For the last couple months, I've been really into Meetup (side note: it's amazing if you're in a new city and looking to make new friends!) and I found a group I really like. A couple weeks ago, we had a large dinner and I ended up being seated next to this guy in his late 30s who I'd never met. I was pretty sure he found me attractive, but I didn't think he was attractive and I have absolutely no interest in being involved with someone who's that much older than me.

I am positive that I don't have a flirtatious personality, but I am pretty outgoing and friendly. I talked to everyone sitting near us, and he would only talk to other people if I was. The moment I was free, he would quickly shift to a one-on-one conversation with me. Once this had been happening for a while, I tried to talk about things I thought he would be bored by (not many men are interested in my passionate love for Sephora lol) but I couldn't think of what else to do without being rude or dismissive. I didn't want to lie about having a boyfriend either. Also, what if I was reading too much into it? At the end of the night, he tried to ask me to go home with him, but I cut him off before he could actually ask by saying "Wow I am SO tired" and immediately ordered an Uber.

Anyway, cut to the present. This guy will NOT STOP messaging me on Facebook. I literally haven't responded to his last 5 messages. I'm hesitant to block him because of the mutual Meetup (which I 100% am still going to go to). I'll directly reject and/or block him if I need to but I'm just wondering if I could've handled this better? I dislike that I've resorted to ghosting.

tl;dr: is there a way I can signal to guys early on that I'm not interested without lying, being dismissive, or embarrassing them? Or will there always be some guys who interpret me being nice-while-pretty as being flirty and interested?