Based on my experience with other online video games proceeding to ban hacked items or people using bots, etc...
My guess is that these would be the most likely reasons to wind up getting your account banned, in order of likelihood:
1) using a hacked car to cheat in an official seasonal time trial or drift trial
2) using a hacked car to cheat or grief in a pub lobby where it's not welcome & people get mad & report you on psn using the "report" option
3) you have hacked cars... and someone, who knows your PSN username (on PSN, here, or somewhere else), feels sour grapes over some perceived slight, getting kicked from your room, or feeling insulted by you for something, and uses the Official Playstation Grief Reporting page to report you to try & get you in trouble - PD checks your acct and finds you have the hacked cars
4) PD institutes some overall system to check game saves for non legit game data, as you log into PSN
#1 - I think is the most likely to get you banned, and soon if not already.
#4 - has obviously not happened yet... but it's possible it will.
#3 is the one that most people here ought to be worried about - based on some of the hysterical nutty messages I've received after kicking someone out of a pub lobby or some of the excessively out of proportion haughty reactions I've seen over race incident disputes.
Not to mention the heated passionate discussions people have over what amounts to liking or not liking green beans.
Also, the internet is not exactly a hot bed of mental health.
None of this matters to me personally because I don't have any of these cars or anything, and I never really cared to either.
However it would be unfortunate I think if someone got banned when they were just messing around with a curiosity for fun, and not actually using the hacked cars to do things that make people annoyed or angry... but said or did something else that upset the wrong person.
So it probably would behoove people to think about these things.
And of course people sometimes use the Grief Reporting system on PSN for legitimate reasons.
Not saying that it's always petty of course.
Thankfully the blocking feature has worked well enough for me thus far. *knocks on wood*
But I imagine the report feature exists for good reason.