If or when he does get back in contact then I need some good ideas for responses. I'll be back for some ideas on how to tell him to fuck off in the best way possible!
The best response is no response. Ideally, block him now as this is the fastest way to actually forget he exists. But if you feel like subjecting yourself to some low-level emotional angst while you wait and see whether he will come back to test your boundaries again, you can do that!
if he does come sniffing around again, the response that will most bother him is no response. Guys who engage in this behaviour - love-bombing then ghosting - are typically some form of narcissist. They are after attention. It’s important to understand that it really doesn’t matter to them whether it’s good or bad attention. Both are satisfying in different ways.
If you let him convince you to bust your boundaries by giving him another chance after he ponies up with a vaguely plausible excuse for his disappearing, that’s high-quality attention plus a feee shag.
If he reaches out and you’re all “Well look who’s back, you feckless asshole, I can’t believe you vanished without a word, what’s wrong with you?” Then that’s also very high-quality attention as he now knows he’s been living rent-free in your head for the last however many weeks. Maybe he can’t get another shag out of you, but the sex actually nowhere near as important to him as knowing that he MATTERS to someone who he can literally take or leave. It’s the emotional heroin of relevance that he’s after. He doesn’t care that you now “don’t want him” as you clearly did at one point, and because you’re so upset he obviously meant a lot to you in the first place, and that means he’s very special.
So complete silence is your best response, as he will be left feeling puzzled and uncertain about whether you actually care that he disappeared or not.
Also good is a very brief “Sorry, seeing someome else now”…. No punctuation, spelling mistake indicates you tapped it out in a rush without much attention paid. Very unsatisfying and will torture him with the idea that he did you a favour by freeing you up to meet someone better than him. Don’t reply to anything that might follow, though you could block him at that stage and he may feel slighted.