I get this a lot, too. Even on Emporer level. However, once in a while, one or two (sometimes even three) other civs will take off and grow into a real threat. It's at these times when I like to start massive world wars, with all civs on the map (some against me, some against each other). This is especially fun if the AI has Nukes. They never seem reluctant to use them. It's funny when like 4 or 5 tiles per turn change into Desert - that's when you know that your warmongering has been a success.
I also like to drop around 20 Mech's and a Settler right on top of another civs coastal luxury, create a city and rush an airport. Let him just TRY and take it back, as I dump another 20 units per turn into the city from the airport.