there's a difference between having an opinion and being slanderous. if you have a problem with a mod/admin you take it up with a higher up, not posting threads to publicly cause flaming matches and potentially discourage members over a personal feud.
should you hate all mods and admins, then obviously you have some biased opinion probably based on one mod (every mod/admin is different, they aren't out on a vendetta against a single person) and are not forced to use these forums. in such case if you continue to use the forums you would be expected to not cause trouble and the best course of action would be to keep grievances to yourself or at least private should the concerns be valid enough to take up with staff.
you cannot be banned for an opinion at all, a mind is your personal safebox which no-one else has access to, so simply thinking something will never get you into trouble.
also the banning is never usually instant unless it is constant harassment or completely overboard.