You cannot make him stop texting you.

You can block him.

I would not though give him (and yourself) hope that someday you may come back. You have learned through painful experience that this relationship is not going to work.

An option is to not tell him that you are blocking him. If you tell him, you will be tempted to want to know how he is handling this news, and he will be tempted to contact you by mail etc. to give you his reaction to this news.