I think if you are only inviting couples you've already found your solution. Address the envelopes to "Mr. and Mrs. Smith," "Jane and Jill Anderson," "Sam Seaborn and Joshua Lyman," etc. Then on the invitation itself say something like "[your name here] cordially invites you to attend a dinner party, Saturday the 5th of November, 2010..." If you aren't inviting single people or people in non-serious relationships you don't need to worry about how to tactfully tell them not to bring a guest, because they will presumably not even be aware that you're having this party.
"You don't just stop posting horse s*** on the web!"-The [Title of Show] Show