One time purchases
I was wondering - if you had a one time purchase in lets say January, but won't be purchasing that item anymore, do you just don't budget for that and change the Availability to Zero? Or do you just delete that category?
One-off purchases rarely are really that. Usually there is (or should be) a larger category. For example, a trip for a wedding seems one-off, but could be included in an "unanticipated travel" category. Drop $10 bucks a month in there and you won't notice it, but it's nice when you need it.
However, to answer the question, options are:
1. Hide the category after ensuring the Available is $0. This will not show up on reports, if that matters to you.
2. If there are no transactions, you can delete the category. This is common for vacations where you save in several dedicated categories (different trips/timelines) but put transactions in a generic Vacation category for reporting.
3. Rename the category. I do this for home improvement. When "Home Improvement-Kitchen" is complete, I renamed it to "Home Improvement-Basement". Reports can be misleading in such cases, so take the specific suffix with a grain of salt.