You can't cancel the pass, monthly payments or not. If you had some sort of customer service issue and wanted to "cancel" I suppose you could complain and they might offer you something, but cancelling midyear in an attempt to not finish paying it off is not an option. Even if you never went back, you'd still owe the balance.
The monthly payment plan, also, last I checked, isn't a financing option. At least, not what I think of as "financing". They're just letting you pay in installments. You provide a credit card and they autobill it. You have to already have the credit to do it. By signing up for the payments you're authorizing them to charge your card on a certain date until it's fully paid. They don't send bills and you can't mail in checks or pay in cash or anything. There's also no fee or interest or anything for choosing monthly payments. The first payment is, I believe, the amount of a one day ticket. The remaining payments are the balance divided by 11. At least, that was the case when I did it. I suppose some of the above may have changed this year.