How to remove a used string

How to remove a used string

Now, I want to remove the string in the .txt file that the user used. This
is to make it invalid for use.



Thanks to Tim Schmelter for the corrected code.
Dim num As String = My.Resources.file
If .txt file.Contains(stN) Then
'Do anything here
'Then I want to remove the string used.
End If