Looking up users in a repeater would be extremely inefficient.
I’m not sure I understand what you are trying to block them from.
Specific posts?
Specific post types?
Something else?
If it is blocking them from a specific post I would add a field group that is only available for you to edit on the posts that includes a user field. You would edit the post and select the users that cannot access that page. This would be a more efficient way to check the current user against a list of users.
