But that won't cause anyone to change their behaviour. Troublemakers will only see "warnings" as a sign that there are no consequences, and won't believe they'll get kicked out until it does happen. There has to be a middle ground between no punishment (warning) and getting kicked out.
You can't just explain abstract consequences to a kid and expect them to react the same way as a rational adult. Some might, most won't.
They'll believe it if they know it has happened to others before. I've found children are experts at figuring out how hard they can push it. They can show quite mature behaviour if you treat them with the respect and expectation of responsibility of adults. The converse is true for grownups that are treated as children.
If you want a middle ground, you might readmit non reincident students, if they insist, after they give some service to the community (not necessarily the school). But you don't present that as a punishment, but as a token of regret and will to change. For that reason, you have to be pretty convinced that it's the child who wants back into the school. You shouldn't listen to parents for this purpose.
You can't just explain abstract consequences to a kid and expect them to react the same way as a rational adult. Some might, most won't.