yeah... how about you let new users only purchase something if they have the amount in their bitmit wallet? I guess that would be the easiest solution
this is actually a good idea and you get to sit on those btcs even longer

So speaking as a customer on BitMit, that would mean I would have to keep bitcoin in my BitMit wallet and that's not something I like to do. Or I would have to transfer Bitcoin twice to pay for an item. That would make using BitMit less convenient.
I personally have only made purchases on BitMit, and I like how its setup and I like the process.
But this is just one geeks opinion.
I don't want to keep coins on bitmit either, don't get me wrong. Only accounts younger than 14 days or something should be forced to use this method.