Set or Delete Property
Use Properties|Set or Delete property to change a property for multiple files/directories at once.
Enter the name of the Property; the drop-down button offers the SVN internal properties for selection. To set a property value, either select Set Value To and enter the new property value, or, in case of boolean SVN-properties, select Set boolean property. To remove the property, select Delete Property.
For directories, choose to Recurse into subdirectories and optionally to Include this directory. Choose Force to skip a couple of checks which are performed for certain property (values).
To get rid of all explicit mergeinfo from your project except from the project root, select
svn:mergeinfofor Property, choose Delete Property and Recurse into subdirectories and deselect Include this directory.