Official comment
You can set their Membership Status to "Disabled". This allows the contact to retain all membership levels but prevents them from logging into them.
This would work in Campaigns using the "Update Contact" action, and drilling down into Websites > website-name > Status > Disabled
In a legacy Rule in Collections, or a Sequence Rule Step, choose "Change the Value of a Field" as the Rule action, then drill down through "Website Subscribers" > this website name > "Status" and select "Disabled" from the drop down.
Comment actions