If I'm guessing correctly what you mean by spoiler effect, I guess a 2-stage election does minimize its effects. For instance, in Brazil, a presidential (or gubernatorial, etc) candidate must have a majority (that is, more than 50% of the votes) to win. Suppose in the first-stage we have candidate A with 30%, B with 20%, C with 15%, etc. Then a couple of months later we have a second-stage with candidates A and B. So it's possible that B wins.

I believe in the approval voting that you mention, there's a situation where candidate C would be able to win, which is also interesting.