Apart from asking them to remedy the problem all you can do is keep pruning the overhanging branches.

What a pain. And eucalyptus can get absolutely HUGE!

Remember that, by law, you have to return what you cut off to the owner of the tree. The law arose because the neighbour was selling apples from the over-hanging tree at prices lower than the tree's owner. The judge ruled that the overhang could be cut down but the material belonged to the owner.

This means you should put the material you cut off over the fence but you could be unneighbourly by piling it in front of their front door.

The owners obviously don't want the cut off material back - to return it would seem quite passive aggressive and will only alienate them.
You have every right to cut back what is on your side. The best thing is to talk to the neighbour and see if you can get on good terms. They are being un-neighbourly by doing nothing about it. This is the sort of thing that can so easily lead to a lot of bad feeling.

in the past we've trimmed stuff back and asked them if they wanted us to dispose of it or did they want the timber for their fire. they were only to pleased to let us dispose of it. they were actually grateful that we were managing the tree as it was beyond their capabilities.