To exclude some strings in sources from being found when scanning with Expert could be used "ignore" tags: {TSI:IGNORE}, {TSI:IGNORE ON}, and {TSI:IGNORE OFF} (for Delphi). To exclude found string just from Found Strings Form you can just delete it and it won't be replaced.
Is this what is needed?
To exclude directly from list should be used popup menu or Delete button on toolbar.
To include any line for scanning in sources which is inclosed in {TSI: IGNORE ON/OFF} block you should "turn OFF" before this line and "turn ON" after this line. This is the only way for this.