That depends: do you hexedit?If not, then the answer is no. There is no method to (legally) remove a character from a party and reduce your party size.
If yes, then there is a bit (or is it a byte?) almost at the very beginning of the party information that designates the number of characters in the party. If you change this from "2" to "1", the second character in your party will suddenly disappear. (For that matter, you can make him, her or it magically re-appear by changing the number back to "2.")