one way to this is to;
Log into your dstv account using the my dstv app or if you have a laptop/computer you can you the dstv self service website selfservice.dstvafrica.com/en-ug/mydstv-sign-in
After successfully logging in, scroll to the section that says try out self-service and click on the manage showmax arrow
You will be led to another page, on this page click on manage showmax
You will then be led to another page and on this page the option to cancel showmax will be displayed. If you still wish to cancel the service, you can proceed.
These steps work if you have showmax linked to your dstv account but if you using showmax without a dstv decoder subscription, follow this link to see the other way you can cancel a showmax subscription