There are three known safe ways to delete restore points stored on your computer.
Disk Cleanup - Launch the Disk Cleanup tool and then select the more options tab. On this tab you will find a section for System Restore. If you press the Clean Up button for that section, Windows will delete all restore points except for the most recent one.
Turn off System Restore - Just by turning off System Restore all your restore points will be deleted. Unless you want this to happen, be careful that you don't mistakenly delete all your restore points by disabling system restore.
Out of storage space - System Restore automatically clears the oldest restore points, if there're no more room for new backups withing the allocated space. It will delete the oldest restore point in order to create space for the new restore point. You can set the allocated space from the Control Panel double clicking on the System icon. Control Panel should be in the "Classic View", use the switch link in the upper left hand side of the window. After you double click on the System icon you should then click on the System Restore tab. Then adjust how much disk space you want to allow System Restore to use.