We were worried about implementing this feature too, citing the same issue. What we did was explain to the users why we were making the change, bless some of the existing users that are known to be in good-standing, and then provide a tiered view of the forum for those accounts with an unverified email address. The people in that group could post only text. However, once their email address was verified, they are automatically put into another group and they can post like normal users again.

So while it's a barrier to entry, we attempted to minimize it as much as possible.