Stuff them full of cat litter until they are next used.
If they went mouldy or slimy and smell musty as a result then rinse them / dip them in a bowl of cold water mixed with milton (other brands available) then dry thoroughly (the stuffing with newspaper works) and the cat litter.
I found Boot Bananas not only didn't work on my rock shoes but actively made them smell worse!
Also remove the insoles if they have them and dry them separately - avoid heat (radiators) because some curl up and don't go back in the boots nicely afterwards.
You can also simply wash them in a washing machine but this can be destructive to the washing machine if the boots are too big and hard and you'll have to reproof leather boots so probably best left to trail shoes and climbing boots. Biological washing powder does a good job. The rubber on rock boots gets nice and grippy after but I've had some (very old) boots delaminate on washing.
Post edited at 20:39