In these cases at least an answer is present, so I need to reward bounty for something which isn't a working solution.
No, you don't. You can just let the bounty expire and it will not be awarded, unless the answer was posted while the bounty was active and has a score of +2 or higher. In that case, it will get half the bounty amount.
I flag answer as "...
If you define 'active' as 'their Last seen date on their profile is long ago', it's relatively simple to figure this out with a SEDE query:
(Keep in mind SEDE is refreshed once a week, on Sunday morning, so if they visited this week, they'd still appear in the results.)