I've only done a quick bit of research on this, but am I right in thinking this is an Android device? Your best bet is to do a factory reset (it's in Settings under Privacy). This will wipe everything and return the phone to its out-of-the-box state. You can then set it up as "yours".
In theory, just removing you aunt's Google account will get rid of the contacts, but a full reset clears everything, including apps and any other settings and will save possible further hassle down the road.
If you've put stuff of your own on the phone, you'll need to back it up first. If you've added contacts, they'll have sync'd with your aunt's Google account, so it would be a good idea to write them down and put them back manually later.
