I’m taken and my guy friend keeps messaging me?

So, I think about putting myself in my SO shoes and would not be fond of him texting a female 24/7, and he’s the same. we have our own boundaries. except this guy who was my guy bestfriend for a few years, will NOT stop texting me. He had issues w his girl who is now an ex girlfriend, and i don’t respond to him and he will text me again 20 hours later about something else on top of the chat i didn’t reply to. I tried telling him that i’m really serious about my relationship and i want to marry him and can’t talk to him all the time. he then was very understanding but he still won’t stop texting me. like why is he asking me about Halloween costumes then showing me booze he’s drinking etc. i’m addition, he has caused issues in my relationship. he went up to my boyfriend to show this girl with her tits out saying why did she send me this, and has created problems with my relationship bc he won’t stop talking and he puts thoughts in my boyfriend ahead. he told my boyfriend everyone on the baseball team wants to fuck your girl randomly. (he is on the baseball team). I don't know what to say to tell him to back off but i need a BREAK from him. does ANYONE know why he won’t stop texting me smh. please give me advice on what to do (i didn’t explain how he’s been a complete problem for my relationship here obv just snippets).
I’m taken and my guy friend keeps messaging me?
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