It's your bookfair (I presume), so you can invite or ban whoever you choose.
My questions to respond to your questions are the following:
Are there to be other non-anarchist folks who will be selling their stuff?
Is this bookfair aimed at an activist crowd?
What's wrong with drawing a line? The ISO is not anarchist, they do not use anarchist organizational methods, do not share anarchist ideals, do not have anarchist goals. They make excuses for Lenin and Trotsky suppressing and murdering anarchists in Russia from 1918-1921. The sooner you draw the line at cooperating with anti-anarchists the better. But that's me.
I have helped kick out some Trots from the SF anarchist bookfair, just as I helped kick out the RCP scum. They are poachers. Let them organize their own events, where you won't be welcome.