[toyg]

> what prevents me

The other side of this rate is that the (central) bank will also charge you for holding that money parked, instead of paying you interest. That plus inflation is (supposedly) offsetting your gain, so you might as well loan out that cash to make real money - which is what the rate-setter wants.

This said, the technical answer to your question is something between "not much" and "nothing at all". Some central banks attach strings to what you can do with that borrowed money, but not all. This is why the effect of negative rates is still debated.