On your schedule page there is an availability editor. Use it to mark the days or the hours you cannot work: the Friday you are at a wedding, the mornings you have a job, the week you are away.
What it does
Coordinators see it when they staff a shift. Somebody who has marked themselves away is not offered for that slot, and a coordinator who assigns them anyway is warned first. If you are looking for shifts yourself and pick one inside a time you marked, you get the same warning — and you can still take it, because you know things the calendar does not.
What it does not do
It does not cancel shifts you already have. If you mark a week away after signing up for something inside it, the shift stays yours until you cancel it. The editor describes your future availability, it does not undo your past commitments.
It is also not a request. Nobody approves it and nobody is told you filled it in; it is simply what coordinators see when they are placing people.
Keeping it useful
Vague availability is worse than none. A volunteer marked available every hour of every day gets assigned at 7am on a Sunday exactly once before they stop being marked that way. Put in what is true, update it when it changes, and the shifts you are offered will be ones you can actually work.