I told this guy to stop messaging me and he keeps doing it. I do t know how to block him. What should I do because he is harassing me in my post. Thx in advance!!!

https://forums.d2jsp.org/pm.php?c=14 block list. You can also block him from the top right of the actual message where it says block user

Block him using block feature for PM's.

Ignore him in your topics, Report it and move on. Ignore the user and it should subside. If it continues, Go ahead and PM a trade moderator with all the evidence you've collected.

Thx

After I block him can he see any of my post?

He sure can

Yes he may still see your posts and you may still see his as the block only applies to private messages,

however if the user is intentionally spamming your posts and harassing you you may report the post and reports will be dealt with accordingly, do not engage him as two wrongs don't make a right.

Thank you have a good rest of your day!!!

You can use the block feature to prevent this person from messaging you. Post #2 has the link