You'd need to manage those legacy users in the DTM user interface.
If you'd like to enforce that all users of DTM be Experience Cloud users, there is a setting that customer care can enable. You can call in and ask them to restrict DTM logins to the Experience Cloud. While your at it, you should have them also enable users management in the cloud. This will allow you to manage all the permissions for your Cloud users in the cloud rather than through DTM.
Can you tell me how to do that? When I go directly into DTM and look at users there is no way to edit them there. I get a message that says:
Please go to Adobe Enterprise Dashboard to manage the list of users assigned to this company.
Click here for more information about user management in Adobe Enterprise Dashboard.
Here is the screenshot. There is no edit button for the legacy (non-marketing cloud) users:
There a configuration which can be reverted to give you access to edit users in DTM and that can only be done by ClientCare. Hence, to get this any of the Supported Users in your organization needs to connect with ClientCare and request this change.
So if you're seeing that message, that means your DTM company is already set to manage users in the Experience Cloud Admin Console. That's a good thing because eventually, all customers will be migrated to using the Admin Console for all their solution permissions.
If you need to edit those users that are legacy, I would encourage a quick call to customer care. They can either remove the users for you or they can revert the setting like Kaushalendra mentioned.