If you expel someone from your territory, you usually place them on a plane to their home country, at which point they are admitted across the border and officially the other country's problem. Most of the time, it is this simple. However, some countries do not want for example terrorists returned to them. So they will refuse those people at the border by claiming for example that they are no longer citizens, at which point they are placed on a return flight to France and become France's problem again (plus they've cost France the price of the flight there and back). France can exert diplomatic pressure on other countries to get them to accept these people, but the final decision is always internal in the receiving country.
Sweden is dealing with a similar issue with Afghani asylum seekers whose applications have been rejected. Afghanistan claims that those people are not Afghani and they will not accept them. There have been a number of more or less secret rounds of negotiation, leveraging foreign aid to Afghanistan among other things, in order to get Afghanistan to acknowledge that they are Afghani citizens and can be expelled there. Without Afghanistan's consent Sweden can place them on a plane to Kabul, but they will be returned to Sweden on the next flight by Afghani authorities. Sweden has no official say over who gets admitted to Afghanistan, just like how France has no say over whether any other county will accept 230 suspected terrorists across their border.
So the right to expel someone is not absolute. You have to have somewhere you are expelling them to, and if no one wants them you're stuck with them.