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 post types?
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.
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