Difficult - I find the old fashioned method of boiling water the most efficient - open up the anthill or nest in the lawn, pour a kettleful of boiling water into it.

I'll cut and paste from someone who asked this exact same question yesterday:

Generally speaking ants are useful in a garden, and form an extremely important part of the whole ecosystem, so I'm not an advocate of killing them off unless absolutely necessary. That said, ants they can become problematic in pots or if they build nests in the wrong place such as your lawn for instance.

Ecocharlie sell something called "No Ants":
http://www.ecocharlie.co.uk/grow-your-own/pest-and-disease-control/nemasys-no-ants.html

which is an environmentally friendly nematode solution that you simply water into the soil and which will effectively drive the ants out of the lawn and the immediate area surrounding it.