Just don't list coins on exchanges until they have over 50% of the hashing power. Problem partially solved (aka ameliorated).
The whole problem is only arising in the first place because people are irresponsibly - some might suggest deliberately, with malice aforethought, which maybe could be argued down to criminal negligence if a judge is in a lenient mood - encouraging people to throw their money away by buying crap that is not even 50% of the hashrate let alone over 50% of the hashrate.
Basically the coins people are complaining about are coins the complainers have not even bothered to adequately secure.
Responsible exchanges should not list such coins.
The exchanges that specialise in listing such garbage are basically just like those "gold game" forums and topsites that encourage the classic ponzi games known as "gold games".
The whole point of those games is to get out fast before you are one of the inevitable losers/victims.
They are in a way a "marketer olympics", where marketers compete to see who has the largest mailing-list of suckers, the marketers earn commissions for referring suckers to such ponzi games plus also some of them actually put money into the games at the start and compete to see which of them manage to get their money back out again before it is too late. If you don't manage to get back at least what you put in you are a loser, a sucker, a "mark", not a champion marketer. But hey if you buy the latest greatest spammer-tools so you can build a bigger list of suckers and get your spam into their inboxes instead of into their spamboxes you might win the next game so step right up folks there are new games starting every day...
Research "51% attack" aka "50+ % attack".
For example of the scrypt coins which has more than half of the scrypt hashpower?
If none, do any two of them added together have half?
As soon as there are three scrypt coins it is mathematically inevitable that at least one of them has less than half the hashpower.
So if you are lucky there might be one that might conceivably be able to be secured provided its miners upgrade to FPGA and ASIC, and there might conceivably be another that might beat it and end up the winner. The rest are garbage, get their hash rate up first before wasting money buying them.
Also, do not be fooled by alternative hashing schemes that use the same equipment.
All coins that are hashed by CPUs are part of the CPU hashpower, which has more than 50% ?
All coins that use GPUs are part of the GPU hashpower, which of those has over 50% of the GPU power.
All coins that can be mined with FPGAs are part of the FPGA hashpower, which as over 50% ?
Only when you get to an ASIC per coin type or per merged mined family of coins do you start to have some chance of maybe being able to secure the coin provided attackers do not buy/own/control/rent more of that coin's ASICs than defenders do.
-MarkM-
you got it. some people think they have the right to walk over a coin and take the BTC's to the bank. middlecoin has almost 9000 megs, they'll take over almost any coin where they go. Once ASIC's come out it will be many times worse