Warn after entering the first new tag that one needs 300 points to create new tags. Don't wait until the user is through filling in the tags and presses Post, before informing him or her that all along he or she was wasting their time, thinking long and hard about what tags to use, when in fact perhaps four out of five might be new tags, and cannot be used yet in the first place.
Note I am not complaining about policies. I am just saying warn earlier.