From the Tools menu, select Autocorrect Options... Then delete the "(r)"
item from the list at the bottom. As an alternative, you can turn off
Autocorrect temporarily by unchecking the "Replace Text as you Type" option
on the AutoCorrect tab.
Note that AutoCorrect is an Office-wide function - if you delete the
entry in Excel, it will no longer work in Word, either (if Word is open
when you delete it in XL, the autocorrect will continue to work in Word
until Word's closed).
You may instead want to simply uncheck the Autocorrect as I type
checkbox.