Interesting pure, I have never heard of that.
Avast+Spybot should work fine for most people.
If you need better virus protection, get NOD32 [Not free]... it has the highest rated statistically speaking amount of viruses caught.
Although I haven't played the game myself, if I'm not mistaken Free Realms is pretty much in the top 3 F2P mmorpgs that are out right now... So it's extremely unlikely that they are going to suddenly close for quite some time.
IP-ban is definitively a possibility, but that shouldn't happen without a reason... I'd dare to at least hope they'd have sent you an e-mail beforehand. I'm not sure why an ISP would block something family-oriented like Free Realms, either.