Hi,
I think so this is not possible, because of the user who created the procedure becomes the owner of the object. As SQL server says, by default the owner will have all the rights to do against the object. Hence he will have the previlage to drop the object.
Unless otherwise the owner of object changed to somebody, he will be having the rights to drop the procedure which is created by him.
If possible you try to change the owner of the object once the user created the object, then you award the permission of exec to that particular user.