Apply disabling of 'Not Going' and 'Maybe Going' to existing events

I'm trying to simplify all the options available to my users and I've disabled the 'Not Going' and 'Maybe Going' options in the back-end category fields.

My understanding is this will take effect for newly created events now... but existing ones (of which I have hundreds) still have the options showing (I understand why)

Is there an easy way (quite comfortable with DB operations) for me to 'deactivate' the 'Not Going' and 'Maybe Going' options from existing events?.. and presumably also to 'clear' the user ID's that have selected those options too.

Thanks!