They now lock down the bounty and airdrop earnings for some period of time. This is aimed at stabilizing the market for such token before the activities of dumpers which are believed to be mainly the bounty and airdrop hunters.
Bounty hunters are not the only problem here. Early investors can also dump the price if they don't believe in the project or their sole intention is making money fast. If the ICO is designed to quickly make some bucks and the project has no long term usability, then they deserve what they are getting. There is no way around this problem I'm afraid.
The real problem is so many icos have offered a lot of bonus to the investors, there are some icos were even offering more than 50% bonus on the presale and private sale and this could be a wrong way that created to many fear about the price of ico after it will be listed on the exchange site.