Idiom/expression that means "canceling" an event from your calendar?

To me, check off = mark an item as finished or done

Cross out = remove or cancel the item

I think, in your case, it'd be "cross out".

The word erased conveys the metaphoric meaning:

removed from my schedule.

When someone asks me to schedule a meeting or event, I often say, "I can pencil you in, but we'll need to confirm details before I schedule this." I don't actually use a pencil, but the imagery of using a pencil implies that it will be easy for me to eliminate the entry from my computer if the details don't confirm.

My calendar was empty, since I'd already erased "Going to Hawaii" from it.

You can axe your planned trip to Hawaii, which means to cancel or end suddenly.

For example, if you have too many activities or responsibilities to take care of, some of them, such as your vacation plans, may have to get the axe.

This is an informal metaphor, and not an expression you would use in a professional or formal statement.

You can scrub your plans, which means to cancel or postpone them. Also informal.

Call off means to cancel, and can be used both informally and formally.

In less delicate company, you can say your vacation plans had to be shitcanned for whatever reason. This might convey your great disappointment about the fact. Shitcan can also mean fire from a job. It can be use to mean disposing of something (that was expected to have some value).

In our brave new world of electronic devices, the word deleted seems like a good candidate:

My calendar was empty, since I'd already deleted "Going to Hawaii" from it.