Well, define "use"?
The only thing that's prevented is SIM based access to the mobile network.
In my example, my O2 phone can access the network over WiFi anyway, so a "blocked" phone could still make and receive calls using my O2 mobile account. Further, it'd still work as a smartphone, accessing my emails, Facebook, personal data, music, photos, there's even a saved account for remote control of my desktop and work computers. A blocked phone in the UK could still be easily exported and sold in other countries outside the EU. Hell, you could easily find where I lived off it and then park outside and leech off my WiFi all day as well. Though I'd probably notice that pretty quick.
Blocking IMEIs is an archaic security system made in the old days when phones could do nothing other than make phone calls. These days, 90% of the usefulness of a smartphone is outwith the actual "phone" functionality.