If someone is abusing the flag system, whatever that abuse may be, it is absolutely justifiable to decline their flag, but determining what's abuse isn't as simple as a hard and fast rule like "if someone flags a question and votes to close it's abusive".
To wit, flagging a question after voting to close is not prima facie abuse. There may be good reason to do so, particularly if the question is egregious and should not wait for 5 close votes. It'd be more abusive if a flagger—who has the ability to vote to close—decided to flag instead.
And in reality, declines happen when a moderator believes it had no merit: either it's clearly not something a moderator can handle, the flag is way off the mark, or a number of other reasons beyond mere abuse.
That is, every flag handled is a judgement call made by moderators based on the specific facts of the situation, and trying to speculate about why a flag might've be declined isn't nearly as effective as the measures we already have:
When flags are declined, moderators are required to give a reason, which shows up on a user's flag history.
It's very easy to just ask why a flag was declined or dispute a moderator resolution on the child meta or in chat.