This is what the shape of the flower and its placement of the stamen and pistils are all about.

If it becomes a problem I would imagine that most victim plants will evolve certain defence mechanisms. For example, some insects will simply chew through flowers to reach the nectar stores - so I think in such cases a flower would just evolve a tougher skin, or secrete something that deters insects from doing that.

great idea... How do you make sure there is an exchange.