I'll get the first response out of the way, if you don't want to go where you are told don't join.
Second, more detailed response. You can refuse, or rather be screened out of a deployment for a variety of legitimate reasons including: illness, severe family hardship, recent return from deployment, and the like. If it is for other reasons you are basically refusing a lawful command and will face disciplinary action that may result in fines, imprisionment, and/or release.
Third response, don't bother joining if the first question that come to you mind is how you get out of a mission.
Fourth response, don't bother joining if the second question that comes to mind is how to quit.
There are variety of ways to release, the most common of which is your terms of service expire and you and the CF go your seperate ways, sometimes with a pension. Otherwise you can request release at anytime, it may or may not be granted depending on: a. your reason, b. how long you have served, c. do you have obligatory service. If you have obligatory service you may still be granted a release but will have to pay back in cash what you have not paid in time.