To delete older restore points, but leave the system restore turned on: Right Click the Drive in question/Properties/Disk Cleanup/More Options/System Restore/Cleanup.
You can also click the Settings button to set a maximum amount of space that you want each drive to use for restore information. If the drive you select isn't the system drive, you can also disable System Restore on a per-drive basis. The maximum amount of space that you can use for restore information is 12 percent per drive.
You can disable system restore and delete all but the last system restore point, then enable it again.