Removing a source from an originality report will 'recalculate' the similarity index without consideration to the removed source in question. This feature is often used when a paper has been submitted twice or more to Turnitin, and the originality report is reporting a high match (eg 100%) to a previous submission. Removing a source from an originality report may provide a more clear similarity index (percentage).
Click on the Assignment link
Click the View all submissions button
Click on the Percentage to load the Turnitin Document Viewer
By default the Filter and Settings will load
Click the graph icon to load All Sources
Scroll down to the bottom of the page and click Exclude sources
Select the sources you wish to exclude
Click the Exclude button
Your originality report will then recalculate (excluding the sources you selected).