If you shut Limewire down and then restart it the history will be erased.
If you don't want the history to show then go to Tools>Options>View>Autocompletion> then untick the box that says "Enable autocompletion of Text Fields" then click apply. And no more history
.
Edit: Also when you start to type a word, the history window opens, and at the bottom of the list it has a "clear history" option.