You can cancel a timer if you know the timer instance id and ask the timer manager to cancel that timer instance.
In general however, it isn't trivial to get access to this information without diving into the details. That is why there is a best practice guideline that helps you managing cases where you need to cancel a timer: instead of trying to cancel the timer instance directly, you just cancel the node / sub-process that the timer is defined in, which will also cancel the timer instance.
For example, if you have a boundary timer event, canceling the node that the boundary event was linked to will also cancel the timer. Similarly, if you use a sub-process that contains the timer, completing or aborting that subprocess will also cancel the contained timer instances.
Kris