When you installed SuSE, you will at least have set up a password for the root account. You can log in with that under the username root.
The easiest way to get SuSE off your computer is to use the install disks; run the installer as far as partitioning the disks, and delete the partitions. Save the partition table to the disk, and reboot (this will leave the space unpartitioned on your disk).
Alternatively, just use SuSE to do whatever it is that you need the space for.