It's not a false statement that a colluded group of witnesses can:

• Censor transactions.
• Block all attempts to remove them from witnesses list.

But if they do so, they would break the network since other users would continue to take as parents the censored units. These users would never see their transactions confirmed and would have to take an action: either use a modified wallet that follows the same censorship rules, either switch to an alt network with a new set of witnesses.