I'm thinking of revoking all of their internet plans
I feel like that's an abrupt decision you have there, have you told your parents about it?
I don't think they would care.
It is wrong to cancel peoples internet plans. They might need it one day and would be harmed if they cannot access it.
If they want to be online, they should pay for it.
As long as you have told your parents about it and made sure they understand that you will no longer pay for their internet, then you are in the right to cut it of as you should not have the responsibility of paying the internet of others. I hope you assist them in setting up their own internet plan in the future if they need your help with it.