I don't think it'd be fair to stop them auto qualifying in successive years.
That'd be like telling Manure they can't win the premiership because they won it last year
Or it would be like telling the best generals, that they can't use the same army codex they used last year.
It is a tricky one though - and I would be quite happy to have a limitation to the maximum number of nominations you can have - at the heats. (though with no limitation on the final - so you could be nominated at the heats, then use that army for a shot at the final.)
Personally I try to self-police myself in this - in that I only use an army a couple of times - but there is no formal restriction to do this.
(my custodes have only been used at the final a couple of years ago - and again at the heats this year. I probably wont take them to another GW event).
Then again - I suppose there is already an incentive to do new things, as people (quite rightly) will not vote for an amry they have seen before and that has already won a prize, so if you want to win another best army award, you need to do something new. (I am planning something new for the final if I can get it done in time.... so much work to do and I started months ago in the summer...)