Could you parse each post for the most common SMS when the 'post' button is pressed? At least one of 'u', 'ur' or 'r' appear in pretty much every post with SMS abbreviations, the poster could then be told they need to fix it before it appears on the forum. That would eliminate frustration caused by posts being deleted. Perhaps an HTML tag could be used to allow 'banned' strings to be posted if necessary?

If the above isn't feasible I'd go for option (3).