The only thing you really can do about such players is call their bluff. The vast majority of the time, that's all they're doing - bluffing, hoping to demoralize you into thinking the system truly won't do anything so that you don't report them. And the vast majority of the time, those bluffs, those claims that they won't be banned, come right back to bite them in the ass.
Just remember, if someone's trolling, here's the best procedure for reporting them;
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Select the "Intentional Feeding" Category. To my knowledge, Intentional Feeding covers all forms of gameplay misbehavior, including trolling and griefing (despite "griefing" being listed as a subtitle for Negative Attitude. I'll clarify that a bit in a minute.)
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Write a concise and accurate assessment of their behavior in the Details box. At first blush, the Details box doesn't really seem to matter much (especially considering the IFS being an automated system, the ability to add comments about a player's behavior seems something like a placebo), I have noticed that I've gotten Instant Feedback Reports more reliably for reporting trolling behaviors with accurate comments.
It's anecdotal evidence, to be certain, but it only makes sense for accurate and detailed reports to be more reliable than general, "tick the boxes and click send" kinda reports. (And, of course, you can comment in the Details box about the player's boasting about not being banned.)
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If you want to take the next possible step up, file a Support Ticket. Support Tickets, while not good in terms of satisfactory feedback (Riot is obligated to withhold details of player punishments for the sake of the punished player's privacy), is particularly good for getting a manual review of someone's behavior. It's a good way to go about it if you don't feel totally confident in the IFS, or if you had forgotten to file a report in the post-game lobby.
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Once all else is said and done, do your best to put the situation out of your mind. After your reports are filed, the case is in Riot's hands, and outside of some certain circumstances, your unlikely to ever see the player in question again, so the only thing you can really do is relax and focus on the games ahead.
Now, to clarify about the Negative Attitude report category; my understanding of the report categories is that, of the three main ones (Negative Attitude, Verbal Abuse/Harassment, Intentionally Feeding), each one pertains to a particular subset of misbehavior.
Negative Attitude is low-key negativity/toxicity, such as defeatism, obsessive complaining, and passive-aggressive behavior, Verbal Abuse/Harassment is direct insults, attacks, and flaming, and Intentionally Feeding pertains to intentional feeding, trolling, and griefing; gameplay misbehavior as a whole, as it were.
So, while it is somewhat of a good idea in general to tick both categories (Negative Attitude + Intentional Feeding), I can only really imagine that, in the case of gameplay misbehavior, only Intentionally Feeding is being referenced as the report category for review, with a review also taking place for Negative Attitude.
Hopefully this all helps to reassure you; I know that with the lack of clarity the IFS has, it can be easy to fall into the trap of believing people who claim they won't get banned, but those people are best just being ignored - they're all bluster and hot air, and likely don't know much at all about the system, much less their likelihood of being caught and punished.
