You need to perform the following search and replace:
Find what:
^STRING1
With regular expression enabled and POSIX regular expression syntax enabled in the general preferences.
I'm assuming from your example that STRING1 does not vary and that it is always at the start of a line.The character '^' indicates the start of a line. Just leave the Replace with field empty.
Please note there is a space after '^STRING1'.
Hope this helps.
Running TextPad 5.4 on Windows XP SP3 and on OS X 10.7 under VMWare or Crossover.