In the Alpha test, no. We hope, perhaps naively, that the numbers will be somewhat uniformly distributed across the different factions and that limiting which faction you may join will not be necessary. If we are proven wrong, we will probably try to consider less intrusive ways of evening the odds between factions, such as scaling the rewards from Zone Control in favour of smaller factions. I'd say that disabling the joining of one faction would only be a last resort. However, it remains to be seen if this becomes an issue in the first place.