I do not conduct stealing others iphones, however if you want to keep the iphone then heres what you do.
put it on airplane mode, this prevents find my device from being used, since you can't remotely force an iphone to put out its signal. Next you take out the sim card, again if you put it out of airplane mode, then the iphone would still be able to communicate with the simcard intact. Taking out the sim card however DOES not make it 100% proof. Now put it in DFU mode, and restore to factory settings. Note you will need a previous backup of an iphone before you do this. Or you'll be stuck in DFU mode/with a locked phone.
Note: this wont work if the iphone is ios7 or above.
EDIT: use this to bypass ios lockscreen
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