For one of the profile who do not have "Manage external Accounts" and do not have "Edit Self-Service Users" permission but still they can "Enable/Disable Partner Accounts".
What am I missing?
Check if the user have some Permission Set that enables him to Manage Users. Check all the permission set of your user who have this "issue".
Go in: Setup->Manage Users->Permission Sets->PERMISSION_SET_NAME->System Permissions->Edit
Remove flag from "Manage Users", in the "Users" section.
Login as the user and check if the button is still there.