You need to keep at least one social login as a way for candidates to identify themselves and start the test. This is to make sure you don't get unidentified test takers and also to prevent fraud.
However, you can choose the email invitation option and invite the candidates yourself directly through Toggl Hire. In this case, you can disable all social logins and invite all candidates yourself.
Navigate to Settings → Landing Page → Edit and untick the social logins you would like to disable.