Proton.me isn’t technically an e-mail provider, that is one of many domains used by Proton AG that mail addresses can be registered with along with others like protonmail.CH, pm.me, protonmail.com, and any custom domain owned by an account holder with Proton AG/CERN.
Your question of how to disable or prevent users from using an anonymous email to register I don’t have a clear answer for, this is kind of a confusing question because any e-mail can be anonymous unless it is directly registered in someone’s name in a public registry (which is required for all .US domains), or they write in their name in the registration form for a discourse forum account which I believe you can set to be required in administrator settings.
If the problem is spam/fraudulent accounts, you can require new accounts must be reviewed and manually approved by staff first before they can access your forum in the setup wizard page two settings:
You can also report proton accounts that are being used for any kind of fraudulent purpose here: