5th columning isn't as simple as preparing systems that do not make a profit. If that was the case, then myself and other Mahon organizers would be responsible for repeated 5th column attacks on Mahon because we've picked systems for other reasons, as are a lot of the organizers at Winters and Hudson for pushing the offensive preps into ALD space.
There is no fast and easy way to determine if something is a 5th column attack. For example, so far this cycle Mahon pledges has put 7,666 merits into DR Crucis. This is a profitable system (81 CC), but it's not one that the Mahon subreddit is interested in. As such it looks to us as a 5th column prep system, and any automated test would fail in detecting it.
The simplest way to fix 5th column attacks is to make preps limited. Each pledge only gets allocated the same amount of preparation goods as they have nominations for the entire cycle. If you're rating 1, you get no say with the power. If you're rank 2, you get 10 nominations and you can pick up 10 tons of preparation materials that cycle. If you're rating 5, you get 250 nominations, and you can pick up 250 tons of preparation materials that cycle.
This is by far the simplest solution, as it requires a minuscule amount of coding on FDev's behalf, it still makes it possible for saboteurs to do their dirty work, but more importantly it also makes it possible to actually counter the saboteurs. Currently the best way to sabotage preps is to build up a massive cash reserve (hello completely unbalanced bounty hunting bonuses) and then spend your week fast tracking a shedload of preps into a system of your choice.
It will also fix Aisling's bonus curse, because it will no longer be possible for trade grinders to completely swamp her prep list week after week by preparing systems with average prices on expensive goods and then selling them for 10% extra in Aislng's space. This change will make it easier for Aisling to actually get back in shape, because shedding bad systems right now will simply result in new bad systems being prepped and then expanded using the exact same bonus curse.
Sandro says he's concerned because it will mean that prep wars will be won by the most populous power, but that's already the case. Currently Mahon will never win a prep war against ALD or Hudson for two reasons. 1) Rank 5 ALD and Hudson pledges only have to spend a few hours to afford fast tracking 10,000 merits and 2) There are far more ALD and Hudson pledges than Mahon pledges, and they could out prep us simply through nominations alone (something we've seen Hudson do before when they've been in turmoil). Changing the current system to my system simply removed number 1 from the equation and means that number 2 is still active. In other words it improves on the current situation.
Fifth columning/sabotage needs to have a place in PowerPlay, just like PvP and player piracy needs to have a place in the regular game.
That being said - I do think it's rather strange that all ten powers are run by blood crazed terrorist supporters, who will gladly pay you 50 million credits for blowing up innocent people, but will charge you out the nose if you want to do useful work for your power (fast tracking fortification merits for example). I'd like to see FDev revisit the merit rewards for undermining for this particular reason, just like I'd like to see them revisit the bonuses (active and passive) and PowerPlay items. For example, why are we almost 40 weeks into the release of PowerPlay with Mahon still being stuck with passive bonuses that do not work along with a weapon that quite literally does not work? You would
think that these things would have been noticed somewhere along the way in QA, but no matter how much we bug report it, it keeps being ignored. Anti-Alliance bias much?

But that's a discussion for another day.