The punchline with this is that 1.) Merchants save ALOT of money. 2.) Consumers will be able to accrue interest on their "spending money" and will have a safer way to carry it around. Read this http://www.mcx.com/ and then remove Fortune 500 club exclusivity from it, along with privacy concerns, and add .4% interest for consumers. BOOM!!
0.4% monthly interest is 4.8% per year and it's tax free for any amount of money . I agree, ppl can make more profit with that amount of money but this is a practically riskless nobrainer investment with better return than most savings accounts.
So I think if you want to mitigate this risk you have to pay lot less POS rewards, probably significantly less than average savings accounts (here in the UK the current annual rates are between 1.2-3.5%).
Edit: BTW how your unsold coins are staking?