There's no middle ground then? It's either come in and read the riot act and issue threats, or do nothing?
Here's a question. You say that areas of the stadium should be closed if people don't toe the line. Cool. What about season ticket holders in that area who aren't guilty of anything? Are you forcing them to move seats? Where are you putting them? Or are you telling someone who happens to sit in that section of the ground that they're out on their ear because someone lit a flare? Season ticket paid for, but no longer valid? That's a nice little court case waiting to happen.
Unless you have some answer to the above that would legally cover the arse of the club, then it's unworkable.
The situation can easily be remedied. You don't aim any messaging or threats at any "groups", you simply say that anyone who is identified as breaking the law will have their details handed to the police. Simple as that. From there, it will be the decision of the country's legal framework to decide what happens, alongside their season ticket being revoked and them personally being banned.