Closing a Lead
You'll want to close a lead that doesn't book with you, whether it's because you were out of their budget or they simple went stale, by choosing the Close button on the right when viewing the lead's overview screen. There, you'll be able to choose the reason they're not booking with you.
Closing a Canceled Booked Job
Just like a lost lead, you'll want to close the previously booked job by choosing the Close button on the right when viewing the job's overview. There, you'll be able to choose the reason they're not continuing with you. This will void the rest of any open order(s), but keep the payments made intact.
Batch Closing Multiple Leads
If you want to close multiple leads at once using the same closed reason, you'll select all or several of your leads in the list and then choose the "Close" option up top. You'll then be prompted to choose the reason for their closing on the following screen.
Closing vs Completing Jobs
Review this article on our Support site that answers this question, going over the difference between closing and completing a job.