Try this, a modified version of Jan's script.
But using "ASObjC Runner" to do a search and replace in the cells.
set TheReturn to ASCII character 13
set TheReplacement to " "
tell application "Microsoft Excel"
set UsedRange to used range of active sheet
repeat with theCell from 1 to (count cells of UsedRange)
set theCellValue to value of cell i of UsedRange
if theCellValue contains TheReturn then
tell application "ASObjC Runner"
set theString to replace string (ASCII character 13) in item theCellValue replacing with TheReplacement
end tell
set value of theCell to theString
end if
end repeat
end tell
PS: Sorry to Jan, for the earlier "direct-reply".