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?
You can play with the User Policy at WebApplication level.
Write powershell script (for exemple) Who add 'All Authenticated users' for all zone of your webAppication with the Deny ALL Permissions for weekend And Remove this rule for working days
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.