This is an issue about the SE network rather than RPG.SE specifically, and has been asked here: Is there a way to disable all forms of notification for a specific site? - Meta Stack Exchange. The long and short of it is: no, not without asking for your RPG.SE account to be deleted.

There are at least two (slightly different) feature requests on Meta.SE to allow muting or disabling notifications, which you may want to vote up:

• Don't show me notifications or comments from sites I'm logged out of

• More control over multicollider

This is an issue about the SE network rather than RPG.SE specifically, and has been asked here: Is there a way to disable all forms of notification for a specific site? - Meta Stack Exchange. The long and short of it is: no, not without asking for your RPG.SE account to be deleted.

There are at least two (slightly different) feature requests on Meta.SE to allow muting or disabling notifications, which you may want to vote up:

• Don't show me notifications or comments from sites I'm logged out of

• More control over multicollider