You will want to use fdisk. This program is in /sbin/ so you will need to be root to see/run it.
To start fdisk, type:
Assuming that g is the hard drive you want to edit, change a is the first, b is the second hard drive in your system, etc.
To view the current partitions on the disk, type p
To remove a partition, type d, then type the partition number you want to remove (can be gotten from pressing p)
For further instructions, try this site:
(yes, it is the installing Gentoo handbook, but the instructions are still good)