we can try to make a tentative and lets see if an user that block another one could avoid that he join a tournament...
personally I have never seen this rule (ok I am not a frequent poker player) but it doesn't sounds fair for the player excluded.
If there is an issue, like harrasment,cheating and so on this must be clear as esclusion and not discretionary by other player.
Imagine, you can add thousands and thousands of players in your black list and you're the first to register in a free roll