[eluser]WanWizard[/eluser]
It depends. If your flood limitation is going to be short (in terms of seconds), store the last post timestamp in the session, and compare that with the current time at the next post. If it is within the flood limit, show an error message.
If the limitation is like "max one per hour", you'll have to store it in server based storage, as a logout/login would destroy the session.