This post lists the steps to clear entries from the Run prompt drop down list.
Step 1 : Right click on the Taskbar, and select Properties.
Step 2 : In the Taskbar and Start Menu Properties dialog box, select the Start Menu tab, and click on the Customize button.
Step 3: In the Customize Start Menu dialog box that appears, select the Advanced tab, and click on the Clear List button.
Now, all entries in your Run prompt would have been cleared. Have fun. 🙂