Rather than limit creativity i would say it promotes it's, by not allowing stocks the onus is on the entrant to use their own imagination and skill rather than that of an other artist.
If an entrant finds a stock they would really like to use in a work the focus then shifts to their ability to recreate that stock using the tools available to Paint.net, this in it's self can be
of great benefit to the artist/user and the Paint.net community as a whole as new and innovative ways to use the available plugins can be discovered, as witnessed by the many tips
, tricks and tutorials that can be found with in the forum. The comment you make is valid, and i fully understand the thinking behind it, however within the confines of this particular
competition 'the piper calls the tune' or in other words 'it's Possums game , he makes the rules'