I want to remove all the access of a Slack user on all of the channels he/she has access, like private channels. Because even if you are admin I can't find any way to remove him if we are not in the same channel. And there's no way I can know where/what channel he/she has access.
As far as I know you can not do that from the UI, but you can certainly create an app that performs this task via the API. Slack has a sophisticated conversations API that allows to query and manipulate all channels of a Slack team including public and private channels.
There is one caveat though. Due to Slack's security architecture no user has access to a private channel, he/she is not invited to. That also applies to admins. So an app like this will only work for private channels if the user authorizing the app is a member of all private channels.
One way to achieve that is to have a generic admin user (e.g. "slackadmin") as mandatory member in all private channel and use that generic admin user to authenticate the app.
We actually have an app our team that automatically and constantly invites / kicks users to private channels based on their membership status and we use the generic admin user approach to achieve that.