It's better to freeze accounts than remove them.
Removing an account removes the evidence that member existed. It removes a part of the community history. It removes a useful lesson about why a member was removed.
It might also remove great material, provocative debate, or simply make previously popular debates nonsensical.
It's not uncommon for the most active members to become the most detractive members. If this happens, removing their previous contributions would be a great loss to the community.
Far better to freeze an account. If you freeze an account you keep previous contributions. You might one day unfreeze the account, if circumstances change. Some members might behave better after some time and reflection.
As an aside, if members ask for their contributions to be removed – make sure you have the mechanism to remove them. Even better, have a mechanism for them to remove any single contributions they would like removed.