How do you stop a WAAAAUGH!(WH40K question)?

[Chipperz]

It is not possible in wh40k universe. Only full scale orbital bombardment aka purging, works. Orks are pretty much mushrooms. Even if you beat the main force and cut them down there will still be spores hidden all over the planet. According to WH40K lore, Orks were created long time ago as a perfect warrior race, that gets stronger and bigger with each fight, won or lost, unlimited in numbers.
The planet would have to be destroyed or drained form all life for them to not be able to re populate.

Man...I hate the new fluff...fucking mushrooms, whose stupid idea was that?

Not to mention it doesn't make sense on ice/desert planets.

Surely some disease/biological weapon could be made which kills the spores/orcs without producing more.

Ice/Desert planets that can also sustain life don't make sense in general. Next?

As has been said, Necrons, Tyranids, Daemons and Exterminatus are pretty much the only way of stopping a Waaagh!. And more Orks. In an old game of Inquisitor I played in, one of the Xenos Inquisitors engineered multiple warbosses to all think they were the hardest, even going so far as to let some of the weaker tribes steal Imperial gear to redress the balance so they'd fight each other instead of the humans on the planet. Orks are most effectively beaten by psychology...