Any good-faith, honest flag of anything marked with the "other" category is fine, and as long as your request is in compliance with the site rules and guidelines and de facto community policies, it will be carried out. That said, you should only use the "other" category when the reason you're flagging it for does not fit at all into any of the existing categories of flag reasons.
But no, you can not flag a user. If you need to report a specific user having bad behavior, you can alert a moderator in Root Access chat.
You can also flag their posts as spam if they're clearly just being a bad actor: if a user gets a lot of posts flagged as spam by many different users, they'll be auto-deleted, and the user will be question and/or answer-banned from the site automatically with no moderator intervention. This is really the best result for everyone: you get helpful flags, the user goes away, the spam gets deleted, and no moderators were harmed in the process :)