Paying ahead is never automatic, so there is nothing to cancel: if you do not want to carry on, simply do not renew. If you turned on autopay, press Turn off autopay on the Billing page first, so nothing more is charged. FleetKraft keeps working until the end of the time you paid for (or the end of the free trial), then becomes read-only.
Your data is then kept read-only for 6 months; pay again within that time to carry on exactly where you left off. Before you go, use Export CSV on the Invoices page and save any invoices you need as PDFs. To close your account sooner, email info@fleetkraft.com.