What should I do if my ex keeps messaging me?

We were dating for about 3-4 months when the quarantine started. The things that started happening between us made me realize we shouldn't stay together, the main reasons were that he was taking it much more seriously and expecting more than I did and he also wasn't mature enough to be on a relationship, which I saw was starting to mess up with my mental health.
So I ended up breaking up during quarantine through message, I don't believe it's the best way, but I also couldn't meet him (I am still on quarantine, so still can't actually). At first he didn't take me seriously, some weeks after he got mad, then he tried to "be friends". I said no because I could see it would be bad to him as I would keep his hopes up to get back together.
The problem is that it has been more than 4 months and he still messages every one or two months trying to get back together. I've been ignoring him the last times since I already said there is no chance for it to happen (and that we can't be friends right now), but it seems he doesn't get it and continues hoping I will change my mind, and this makes me feel bad for the hole situation...
Is there something I should do that will make/help him understand? Or just keep ignoring? PS : I worry about his mental health.
29 d
1 - I know I can block, but he was the first one to block, so before sending the text through it I couldn't block him. And it's not police case...
2 - I'm not a drama queen, I want him to stop, but I care and I don't want him hurt for so long. Empathy, is that so difficult to understand?
3 - He has a historic of mental problems, that's why I feel I need to be careful.
4 - This 3-4 months I've mentioned were "real dating" "in real life", if it wasn't clear.
29 d
I'm already following my friend's advice to not answer him, my question was about if it was the best way to handle the situation, I just wanted to get more opinions...
What should I do if my ex keeps messaging me?
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