I think this maybe isn’t as large an open goal as it first sounds. 
Mostly, TL1 is a reasonably brief phase for most people (default 15 visits and some light participation), so the window of opportunity to do this isn’t that large before they become TL2.
But to hide a post it needs to accrue enough ‘points’ to push it over the hide post threshold (as admin/mod, you can see the reviewable score for a particular one by clicking on the question mark icon in the top corner of each reviewable in the queue). For low trust users they get a lower point boost than higher trust levels (only 1 point for TL1) so you’d need more flags from more users to build up enough to hide a post.
There’s also a user’s flag reputation as well, which has a direct effect on whether it adds or subtracts points based on how many previous flags have been agreed/disagreed with. So if the mods are disagreeing with all the flags these TL1 users are submitting then their reputation should tank and future flags will have even less chance of hiding anything.
This topic has some info in about how the user reputations work (and hide sensitivities too if you were looking to change something on your own site):
Also, it’s worth noting that the ‘something else’ flags don’t trigger a hide post.
I think normally when topics crop up about disingenuous flaggers it’s more about them wasting staff time rather than having too much of a public impact. Though the ‘hide post threshold’ is a dynamic number based on some magic under the hood, and I think I have seen at least one example of it being unintentionally super low which did mean it was more of a hair trigger than it was intended to be.
But if everything is working as intended then there are already some reasonably sophisticated safeguards in place. 