One way: Replicate your mail to local. Set yourself to offline mode. Read your mail. Go into mail.box (it's a db) and delete any outbound receipts. Set yourself back to online mode (original location document).
Another way: (may not work w/ all versions of client) View documents in preview mode. Make sure you don't have mark docs as read when previewed in the user preferences.
Another way: Create an agent to run on all your incoming mail use forumula:
FIELD ReturnReceipt := @DeleteField;
-> You could modify your inbox view (create a new view) to have an icon for mail that has a return receipt and then run an agent on just those messages if you don't want to disable all return receipts.
...search is your friend.
That said, It defeats the purpose of having return receipt option if people can disable it, so, it's not supposed to be disabled. Sometimes though, it is nice to be able to prevent it from going out.
Ciao.