Remove Specific Text from Multiple Descriptions
Consider using an AppleScript to perform this task. The following AppleScript will get you started:

tell application "iPhoto"

    repeat with i from 1 to number of items in photos

        set myPhoto to item i of photos

        set comment of myPhoto to replaceText("find this", "replace with this", comment of myPhoto)

    end repeat

end tell

-- replaceText from Bruce Phillips (http://brucep.net/2007/replace-text/)
on replaceText(find, replace, subject)
    set prevTIDs to text item delimiters of AppleScript
    set text item delimiters of AppleScript to find
    set subject to text items of subject

    set text item delimiters of AppleScript to replace
    set subject to "" &amp; subject
    set text item delimiters of AppleScript to prevTIDs

    return subject
end replaceText


To use this AppleScript, copy and paste it into a new AppleScript Editor document.

In this situation, you want to find and remove a specific piece of text. Modify the above AppleScript as follows:


Change the "find this" segment and enter the text to find.
Change the "replace with this" segment to be blank, "".


With these changes in place, save the document, and click Run in the toolbar. Depending on the number of photos, this AppleScript will take a long time; potentially hours.