Documentation on Triggers?

It is set that way (only in backend), but a user who has had that plan still gets the option to reactivate it (and presumable to renew it, if the subscription is active) in the front end.

Only way to turn off renewals on frontend is to disable renewals entirely by setting "Renewable in advance by" to "Non-renewable" under the Pricing tab.

That turns it off in the back-end also. Is there any way to be able to reactivate/renew in the back end but not in the front end?

Maybe I should explain a bit more. Following your earlier suggestion, I have a link on the renewal page to where members should renew with volunteer hours instead of payment. This is in a custom component. The process now is to handle the hours there and then manually do the renew in the back-end. Is there a better way to do the renewal?

There's no way to disable renewal exclusively on frontend at this time. Renewals also can not be conditioned so there isn't a way to condition them based off external information (e.g. fields) at this time. I'm not sure what to suggest for your specific situation as you're basically needing conditional renewals, which isn't implemented yet.