It's not quite that simple. The RIPE database stores mostly administrative information, and doesn't _directly_ affect Internet routing.
In order to "steal" IP addresses (get them routed to you) you would need to buy a connection to at least one exchange point, probably several if you want all the traffic for the target to route to you and not just some traffic from some networks. You'd need to buy rackspace somewhere with a connection to the exchange point, install routers, establish BGP peerings with the exchange point (if they're doing route reflection) or with all the other major networks at the exchange.
There are multiple steps along the way where humans would look at the prefixes you were going to be announcing. This would include looking them up in RIPE, but anything more than a cursory inspection would likely reveal your ruse.
At this point it becomes more of a social engineering attack, and even if you got as far as announcing it, there are things like BGPMon that would pick up the fraudulent announcement pretty quickly and you'd likely find that the cable was pulled out of your router pretty fast.