How can I get a guy to stop dming me?

So this guy I met awhile ago - haven’t had much interaction with him but I always thought he was weird but harmless. It has been a period of time but me keeps sliding into my dms - when I post a story he always comments- I was trying to be friendly but - he took and inch and ran with it. I think he caught feelings - (we literally had two conversations that were vague ) I stopped responding and he still won’t give up - he still messaged me. it has been two months of this- I am friends with his sister - so it’s tuff! Please help! What should I respond in a mature way?
How can I get a guy to stop dming me?
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