If you have no further use for your VPS, you may cancel it from the control panel at any time. For pre-paid servers, your VPS will remain active for the remainder of the paid subscription period.
For hourly billed post-paid subscriptions, once cancelled, your VPS will shut down within 10 minutes, and you will receive a final invoice for the subscription within the following weeks.
VPS subscriptions can be cancelled in mPanel right here https://www.mammoth.com.au/mpanel/cancel-vps
The animation below demonstrates the VPS cancellation process