Sorry, you can't ban US citizens moving within the US.

We can, however, seriously slow down US immigration (legal and especially illegal), which is where about half of the inflow to Colorado is coming from.  Our population would still be growing (from US citizens moving here from other states), but not as fast in total.  

Personally, I think 320 MILLION is already more than enough people than the US really needs.  We're now the 3rd most populous country on the planet, and rising fast.  I really don't want to catch #1 or #2.