Open the machine and press Archive this machine, at the bottom under the notes.
It comes off the Machines list and out of every place that offers kit for a job: the job and contract kit lists, the crew's yard and fuel screens, the quote machine list. Nobody can book work with a mower you sold in March.
Everything it did stays exactly where it is. Its service records, LOLER and PUWER checks, fuel, faults, the finance agreement and insurance certificate you filed against it, the hours it ran on last year's jobs — all still there, and last year's job costs still add up. That is the difference between archiving and deleting, and it is why archiving is the one you want.
Changed your mind? Machines → Archived lists them, and the machine's own page has Put it back in the fleet. The only thing that doesn't come back automatically is the kit lists it was on, because nobody remembers which crews wanted it — add it again where you need it.
Hired kit that comes and goes is worth archiving between hires rather than deleting and re-adding, so the check and service history builds up over the years instead of starting again each spring.