Very often on my forum register Users who aim to write a post with a hidden advertisement (link to another page).

Is there a modification that removes links in the posts of people who have recently registered? I can not find one. 

Kind of. Just moderate every first post. 

I thought that someone had a similar problem and there is a plugin checking the number of User posts and if less than x blocking the string http letters

If they can’t post the links directly, they will post phone numbers, show URLs like www,spamwebsite,com and so on. If you let them post freely, they will post spam.