Is there a way to block the use of SharePoint for multiple users on the weekend and allow them to enter only during the work week?
This is a bad solution, but only one that comes to my mind without ADFS.
You can remove their permissions by removing security group and then re-adding the group. SharePoint Online groups and users can be removed and added using Powershell, so you could automate it into a script running at certain hours. Please check:
Adds an existing Office 365 user or an Office 365 security group to a SharePoint group.
Removes a user or a security group from a site collection or a group.