I'm down for protection, I gotta grip! While studying up a bit I came across this:
http://answers.yahoo.com/question/index?qid=20081118193958AADwsKn
it is geared at the US but I think it probably applies most everywhere.
I like the chemical dispersant idea. But it may contaminate what you are protecting. But a dye pack, or
http://www.predatorpee.com/ this product would wash off your crop pretty easily if caught quickly. But the poor fucker that gets into it wont smell right for a while!
Spider/mouse stickies. The stickie pads, they would be detoured on the stairs if they got into that shit!
Rig the house to light up upon entry, like a motion sensor rigged to extension cords going through out the house, and an alarm, whistle, etc.
Any kind of noise maker with a barking dog! Reinforce "good dog" for un-knowns or potential problems, and "quiet!" With known causes, deer, squirrel, Grandma Neddie, etc.
This way, even if they still bark at "false alarms" it will take on a different tone! My dog, C has a yappy excited bark for pheasant, constant bark for deer/cattle, goes ape-shit for unannounced visitors, but silent for ground squirrels, so she can catch 'em!
My FAVORITE, a dog that you don't intro to anybody but close partners. Then nobody kbows what they are up against. Even "buddies"! Most rippers are from your own crew, or extended associates. So, if your buddy knows of this other dog that "cannot come out, he's not good with people" he will make your place less of a target. Just a barking dog usually "sends 'em packin"!
Don't rely too heavily, and always protect them from assault! (Loaded meatballs, dangerous animals etc.)