[Logician]

g-force2k2 is right.

You cant trust their IP because most of the users have dynamic IPs, so everytime they connected to the internet, their IP will change.

You cant trust to cookies either, because some may disable it, or can disable whenever they want to pass your restriction.

So even though you design a system for this, it would be quite awkard and not very efficient.

One solution would be to restrict guest posts to X posts in Y hours but this restriction cant apply to a specific guest but all guests instead. For example if you limit it to 5 posts per day, guest X can post 5 and no other guests cant post anymore for that day. A basic IP check can work to restrict guest X post to eg.1 only, but this again wouldnt be very trustworthy for the reasons I mentioned above.

I suggest putting the guests' posts in moderation if you cant disable them alltogether. Most efficient solution IMO..