Ahaha, Caith, I think if we forbid the people to write on the month they pick the theme, we're turning this prize into a punishment. I would either pick fanfic each time, or flat out refusing it. If you force me out of a month, no guarantee I'll be back for more. Newbies also need the encouragement, and the boost in ego and confidence that winning your first month provides would be totally crushed by not getting to write the next month as they pick a well loved theme.
Besides, I think the worry is rather pointless. We all love scifi and fantasy. Plenty of the themes worked great for everyone. I've had some months where I was super inspired, loved my own story, and was a bit miffed not winning, but others did love it too and got it better than I did.
On the other hand winning the smut month was... unexpected and not my proudest achievement.
I genuinely think that banning anyone from the contest is a bad idea.
Also not sure about a 6 months long thing... I mean, I'd not enter it. It's really hard to keep tabs on monthly shor stories besides a long one, and it's not like we could stop the monthly while a multi-month one goes on. I mean, nothing stops you from setting out up as an exercise with interested people. But if we stopped the monthly for 4 or 6 month, any newbie waltzing in would not be able to join easily and it would completely defeat the point of attracting new participants.