How can i set permissions so only certain groups can delete posts?
Edit the usergroup, select the Permissions tab, untick Can delete posts.
But super moderators can delete and possibly i think moderators can too but there is no options to stop them from deleting.
Super moderators are super moderators. They have full global permissions, as far as I'm aware.
As for moderators, you can edit their options in the ACP and untick Can delete posts?.
It only says "can delete own posts"
There must be away of adding something to stop all members except admin from deleting posts.