Except from discarding this e-mail address, start with a new one and keep it as secret as possible. That doesn't guarantee that nobody will try to spoof it anymore, but if it doesn't exist anymore you won't notice it's being used.
It seems someone is using my Protonmail email to spoof. I've now gotten 2 emails the look like they are to/from myself noted by Protonmail with this message: "This email seems to be from a ProtonMail address but came from outside our system and failed our authentication requirements. It may be spoofed or improperly forwarded!"
Can I do anything to stop this?