Ha! You wish.
First of all it's quite possible to modify the supposedly immutable IMEI of a handset. The networks allow any valid IMEI on the network as long as it's not on the blacklist.
Secondly, you might THINK you've blacklisted the phone with the network but what they don't tell you is that they within a week or two that blacklisting will mysteriously evaporate... I have this on good authority from several of the employees at a Vodashop - they often blacklist handsets for customers but when they check up on its status a while later find that it is no longer blacklisted.
This is of course because it is absolutely in the networks interest to have as many phones stolen and repurposed as prepaid handsets - more customers! More phone sales (to the original owners)!
We had a short term cell phone rental business once and always recovered our own phones. We never blacklisted them but let them go back into circulation. Then, after a few months and once there were a number of stolen handsets in circulation, we'd get a warrant from a magistrate to get the SIM records for those IMEI's from the networks. We hired a voice artist to call all the new "owners" and lie to them - telling them what a great prize they've won and then easily getting all their personal particulars (place of work, etc). Then we'd do the rounds with a police constable and simply go pick up all the phones. Protesting users were threatened with prosecution for receiving stolen goods - that, together with the presence of a copper quickly convinced them to part with their ill-gotten goods.
This had the added benefit of dis-incentivising people from wanting to buy stolen phones again in a hurry!
PS. Have you ever wondered how you can sign up for R11.70/month to
track any phone with the owner's permission (i.e. it's really easy for the networks to offer this) and get positional accuracy within about a city block - yet even if you've reported a phone as stolen at a police station, the networks are somehow unable to track it's position for you? A friend had her laptop bag stolen once with a switched-on cell phone inside and we thought we could convince the network to track it - but they wouldn't do it without a court order... yet I can transact to the order of 100,000's of rands with my bank - via the phone!?
Again, the obvious answer is that the networks do NOT want you to recover your stolen phone - they want someone else to start using it, and for you to buy a new one...