Bought used iphone but has service from old user?

So I just bought an iphone 4 cdma (no SIM) from this dude, he has Sprint. Calls, web, his "sprint 3g" data works. Gets aroudn 2.5 mbps btw, but what should I do? It's only been one day since I had it so if he terminated the acc it could stop the service at the end of the month, but I want to make sure I'm not messing with his data limits etc.etc.

There's no reason to wait until it's not getting service, since you have to activate it on your own account anyway. Once you do that it won't be on his service anymore.

P.S. Since it's CDMA it'll only work on Sprint, or possibly with some other smaller CDMA carriers that can "flash" a phone.

This is why you check the ESN before you buy a phone.

No phone should be activated upon selling. Deactivation and activation are instant. No delays