Didn't spot this, and I imagine you've probably solved it by now, but essentially you should be able to drop the manually applied styles by converting from a text object to a plain string and back again.
Code:
tell application id "OOut"
tell front document
-- WHICH OF THE COLUMNS ARE RICH TEXT ?
set lstColID to id of columns where its type = rich text
-- WITH SELECTED ROWS, FOR EXAMPLE
repeat with oRow in selected rows
repeat with strColID in lstColID
-- EXTRACT THE PLAIN TEXT, AND RESET THE TEXT OBJECT
tell cell id strColID of oRow
set strText to value
set value to strText
end tell
end repeat
end repeat
end tell
end tell