This will depend on the version of Crystal that you're using. I'm not sure when the ability to manipulate memo data in a formula was added - I think it was either version 9 or 10, so this will probably work for that and XI.
You'll need to create a formula that will replace the carriage return (chr(13)) and line-feed (chr(10)) characters in the memo field with a space. You'll then use that formula on the report instead of the memo field itself.
Replace({table.memofield}, chr(13) + chr(10), " ")
NOTE: You may have to tweak this if your memo field only contains carriage returns instead of the cr/lf pair.
-Dell