Wiki - The paradox of tolerance states that if a society is tolerant without limit, its ability to be tolerant is eventually ceased or destroyed by the intolerant. Karl Popper described it as the seemingly self-contradictory idea that in order to maintain a tolerant society, the society must retain the right to be intolerant of intolerance.
I've always disliked how this is described as a paradox. It only highlights a broader point found in many systems, a just system is never about "the good" outnumbering "the bad". It's about a balanced equilibrium, as are most relationships. Besides, allowing intolerance is not a tolerant act, that's not the way we define that term. To make such a claim would be as ridiculous as a racist person saying they are practicing tolerance by not challenging or question any of their bigoted thoughts and instead just letting them play out.