Religion, time and blackmailing
Have your world feature a religion that forbids the use of certain kinds of magical practices that would use your loopholes. The in-universe explanation would be that the church or whatever institution you want feared that people were too powerful and therefore uncontrollable, so they invented reasons to not do this thing.
They also changed the education so that new mages wouldn't find out about this or are trained to avoid situations that would lend themselves naturally to exploitation of this loophole. If every authority you knew said "Don't do this, you will die!" then you won't do it - even if it wouldn't change a thing.
If someone still tries to do this and succeeds he will probably talk about his great discovery. Which will lead to someone important hearing about a wizrad that will soon be uncontrollably powerful. Powerful people don't like people that are far more powerful than them and not under their control by other means. Either they will swiftly be executed or you could blackmail the wizard into following the standard rules and keeping quiet about his discovery if he doesn't want his family to suffer.
I first heard about this method in The Black Magician Trilogy by Trudi Canavan.
The wizards have a certain school that teaches everyone little magic tricks that use their life force. Wizards are problematic because their life force violently explodes when they die. But for some reason there are very, very old graves nearby and nobody knows what that's about, because there is nothing left after a wizard died.
Later you find out that somewhere else people still know how to absorb the life force of other people. Basically the master teaches the student and therefore gets a regular supply of their life force, absorbing parts of their magic through open wounds/their blood. This way they become incredibly powerful, especially if they have a lot of students.
Normal mages are nothing against these people and once they find out that the first group has forgotten how to use this magic because they thought that "absorbing other peoples life force is dangerous and cruel" they quickly attack - and nearly wipe out the first group. Except for the main character of course.
By using a method like this you can explain the loop hole and even use it for social interactions about why someone uses this loop hole or not and what the problems are with it. It gives you more variations and always an easy way out like "Don't do it - you will get more problems than you can solve". Which the main character does not necessarily have to follow, leading to the next story arcs where he has to figure out how to clean up the mess he created by using this loophole.
No need to close the loophole - just place something in front that says "Loophole ahead - be careful!" and let your main character/villain/... be everything but careful.