How was racial violence used to deny rights to african americans

As an intimidation tactic.

Think about it:

If black kids are beaten up for going to school, will they even want to go to school?

If a black man was going to go vote, but the night before, the local sheriff came by and told him he'd "cause trouble" for him and his family if he votes, will he still want to vote?

Basically, violence was used to scare black people into obeying.