As you can see, the switch doesn't have an OFF position, it diverts the current either between middle and left, or middle and right contacts.
Had to attach it to a wall lamp that didn't have a switch, by drilling a hole in the base of the lamp:
I have therefore one contact that I am not going to use and where I don't want current to flow at all. I cut it off and rolled all over the switch body in the place with 2 layers of insulating tape. The remaining 2 contacts were rolled in a layer of insulating tape as well, after connected to incoming live (middle contact) and lamp live (left contact) wires respectively.
As I drilled the hole in the base of the lamp, it turned out that the side of the switch with the spare contact may be touching the drywall a bit.
The question: since the current is supposedly trying to go from middle contact to the spare contact when lamp is turned off -- is it liable to cause problems such as heating up its insulation or similar -- simply by being live?
I am grateful and have upvoted all the three current answers, each of which provides relevant points for me. I feel that all three taken together answer the original question practically fully, but since I can only accept one, I gather I should accept the one, which most directly addresses the original question.