NTLDR's solution is correct (as usual) but I guess this would stop deleting via "admin commands" only. Mods can still delete posts by "edit post" option.
And what if you also hacked that section? They can then edit the post and remove its content by eg. ".". Is it so different than deleting?
I agree with Xenon here: If you dont trust them for deleting posts you should really think twice why they are mods anyway..