At BLOX, you are in control of your coins. You can stop Earn at any time. This is because at BLOX, your coins are not 'locked up' (as with trading exchanges).
So your coins continue to be tradable even when Earn is activated. If you decide to sell the coins, you can do so at any time.
If you activate Earn for a specific crypto, all your coins will be lent out. If you buy new coins while Earn is enabled, they will also be automatically lent out. The same goes for the returns you earn. So you will automatically earn returns on your entire investment in a particular coin.