Waiver. Although not required, your agreement may include a provision near the end (sometimes called "waiver") that states that your failure to exercise a right in one provision doesn't mean you are waiving other rights. The Dear Rich Staff also suggests that if you're concerned about this, you can preserve your position by sending a letter or email to the publisher saying something to the effect of "I know we didn't hit the royalty target in the contract but I would still like to continue our arrangement. By not canceling now, I am not waiving my right to cancel if we miss subsequent targets."
Termination. Are you sure the agreement doesn't explain how or when to terminate? Usually the publishing agreement has a section entitled 'termination' that explains the hows and whens. If there are no such requirements then termination is probably okay in any manner that provides reasonable and prompt notice of your desire to end the agreement.
This Archery Target ... is reminding us of when we were at summer camp and we got an archery award ... but then things went all swirly when we found a link to our old camp and got super-depressed. Was it sold to Six Flags or something? Water slides, dirt bikes, bungee jumping, four-wheeler ATVs, spy camp (spy camp?), and skateboarding -- and who are those kids with assault weapons and face masks? Oh well, thanks for the memories!