Prevent IMEI or Serial from being blocked?

Hi, I just registered here as a horrifying thought hit me. My GF just bought a 2nd hand 4s from a 'Cash Converters' here in the UK. Its working fine for now, but a friend of ours had bought one before christmas and it ended up being blocked, now its just a fancy pants ipod touch!

Are there anyways to prevent this from happening? can i change the serial or IMEI via jailbreaking app etc..?

I dont trust Cash Converters, i had sold them an xbox 360 a few years ago and instead of them formatting it (as they say they will) whoever bought it had my gamertag and Visa details and started buying MS points! (yes, yes i know, i should have formatted it myself doh!).

So, can i change the imei/serial or am i better crossing my fingers and hoping it doesnt get blocked?

many thanks

DM