Removing annoying prompt

I've recently purchased the DLCs on sale, and boosted my new character to level 80. However I keep getting an annoying prompt telling me to level up my mastery. The problem is I need to complete certain a story mission to unlock my mastery, but I've yet to complete the main campaign, so I'm reluctant to look into it to avoid any potential spoilers. Is there some way of turning off the prompt? It's seriously popping up every time I earn experience.

No but if you have a guild with heart of thorns or path of fire guild hallI think you can travel there exit into either expansion and unlock mastery tracks that way.

This happens even when not using a booster and it's ridiculously annoying indeed. You will find some information there:https://en-forum.guildwars2.com/discussion/8524/mastery-popup-will-not-go-away/

Thank you for the suggestions. Looks like it's been an ongoing problem for years and they've refused to fix it. I'm not going into the expansion areas without first finishing the core game, as it looks like there's multiple things I'd need to do there to get rid of this pop-up. I'll just have to deal with it, and hit them in their wallet by playing the game without purchasing any gems. They probably won't care but to me if feels like I'm getting back at them.

You don't have to do multiple things.
You can join someone else's PoF 'Sparking the Flame' story instance and turn off sound and close chat. When you get to the Renown Heart, just finish it, and voila, you have a Raptor, and Mastery Tracks. Or, as mentioned above, join a Guild with a Guild Hall and visit it; if that doesn't stop the pop-up, just gather a couple of Mastery Points, or use any you have already gathered. There's several very easy ones in Core Tyria.

Good luck.