It depends on the stage of the game as to how you'd go about it and what type of player they are..
If its near the start of the world, restart letters work sometimes, otherwise catting them down is also good, this can also work around this stage of the world, catting down someones villages makes them loose intrest when theres nothing they can do. Depending on the Player, Nobling them would work as well, some people, not naming any Kopouts here but they hit the delete button after they loose a village or 2, others will fight down and the only way to be rid of them is to rim them.
If your the larger player, get them to make it easy for you, agree to work with them and then smash them while there troops are out, if you prefer a more noble option, cating works wonders after you've cleared them although that should leave you open to Noble the village unless you need the farms..
Hope it helps..