What’s the best security control against this robbery?!

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.

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

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