Hold command r keys and boot the computer into Lion Recovery Partition.
From there you can erase the Lion OS X Partition using Disk Utility and download and install a new copy of Lion from Apple's Servers.
Upon rebooting normally you will go through the setup and can set whatever name on the machine as you wish.
All files and third party programs will be removed, so save your files off the machine and resintall programs from fresh sources, download iLife from the AppStore again if you need it.
Sorry this is the only way it can be done to remove a previous owner entirely from the machine, you can delete a use from the System Preferences> accounts, but not the owner unless you do a total reinstall.
copy from another post...
I would strongly advise you not to do this. However, if you insist, Apple have provided instructions here:
It's a fairly old page, though it looks as if it has been updated for Lion. Back up before proceeding.