Hello,
Thank you for contacting us and bringing your concern to our attention.
You can disable the 404 Monitor from WP Dashboard > Rank Math > Dashboard > Modules:

Please note that some options are only available when you enable the Advanced Mode in Rank Math. If 404 Monitor is not showing, please switch to the Advanced Mode: https://rankmath.com/kb/switch-from-easy-to-advanced-mode/
Hope that helps and please don’t hesitate to let us know if you have any other questions.
Hello,
Since we did not hear back from you for 15 days, we are assuming that you found the solution. We are closing this support ticket.
If you still need assistance or any other help, please feel free to open a new support ticket, and we will be more than happy to assist.
Thank you.