Blocking domains through the Azure AD B2B collaboration policy stops group owners adding new guest accounts from certain domains. It does nothing about existing guests from those domains. Fortunately, it’s relatively easy to check the guest membership of Groups and Teams to find guests from the blocked domains. And once you know those problem guests, you can decide what to do up to and including removing guest accounts from the tenant.
Azure B2B collaboration is used by Microsoft 365 Groups-based apps like Teams, Planner, and Yammer to control the creation of new guest accounts. You can update settings in the Azure AD portal to stop new accounts from specific domains or restrict guests to a list of known domains. But before you go ahead and update the settings, it’s a good idea to know where existing guest accounts come from. It’s easy to create a report with PowerShell. The next step might be to remove guests from offending domains.