How do I tell an ex I don't want to be friends since ignoring him isn't working and I can't block him?

So my ex has texted me since we broke up but I've been ignoring for the last month for a couple reasons.
First, anytime I reply he just argues with me about what I say or if it's about him he gets defensive about why he does things the way he does things. And many times he's argued with me about things I definitely know more about but has argued with me anyway.
Second, I'm in a relationship and, though I find no problem with people being friends with exes to an extent, I don't want to talk to him since he's flirted and tried to make me jealous I think.
Third, and the most tiny problem, he has no social skills. Even in the five months we dated, he barely asked how I was, and since we've broken up he's asked once since we had one of the recent hurricanes come through, and he only ever talks about himself.

Help?
How do I tell an ex I don't want to be friends since ignoring him isn't working and I can't block him?
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