Not really viable. If you fail to complete your ticket, the airline has the right to reprice the flights you did take as a series of one-way flights, undoubtedly much more expensive than the cost of your DONEx ticket. If your situation is that you don't need the remaining flights, you may want to fly them anyway, both to earn the extra miles and to avoid oppressive repricing by the rules. If you can't fly the remaining flights, I would suggest changing the dates to as far out as you can then just don't fly them. It's very unlikely that the airline will realize that you never completed the ticket, and anecdotal evidence in various FT threads confirms this.