To cancel active plug-in monthly payments, first sign in to The Collective website and go to your account page: https://collective.focusrite.com/my-account/
You can see which plug in you are actively paying for, to cancel the monthly payments for any single plug-in click "Cancel monthly payments" and then "Yes, Cancel monthly payments."
You'll be asked to provide some feedback and then be unsubscribed from your monthly payments and receive an email confirmation.
In your account, you'll be able to see the new status:
Your plug-in will still work in your DAW until the term ends, at which point you'll only able to use it in Demo mode. When you resume your monthly payments you'll continue contributing to the total purchase price.