How might I cancel scheduled downtime on Nagios

Solution 1:

In the left hand column of the Nagios interface (underneath "System") pick "Downtime". That takes you to a page listing all scheduled downtime and lets you delete them individually.

Solution 2:

Along the left-hand side, under "System", there's a "Downtime" link; it goes to /cgi-bin/extinfo.cgi?type=6. Once there, click the trash can icon for the downtime in question.

Coincidentally, you may wish to investigate the difference between "scheduled" and "flexible" downtime, or better yet, given your use-case here, an "acknowledgement" sounds like what you really wanted in the first place.