I had a big infestation of plantains growing in the grass in my last garden. They are actually quite easy to deal with as they are very susceptible to weed killer. If they are in the grass then just spray them with lawn weed killer - the kind that kills the weeds but not the grass. If they are elsewhere then spray with glyphosphate. They will give in easy.
Plantains are easy to get rid of if you don't mind using weedkillers as Alice says. This stuff will do the trick: http://www.lawnsmith.co.uk/shop/moss-weed-killers/weed-killer-concentrate/vitax-lawn-clear-weedkiller/
If you don't like weedkillers it's quite therapeutic to dig them out. They come up easily and considering the amount of space they take up in the lawn when out you wouldn't even know they had been there.
if you don't want to use chemicals on your lawn, I use a small amount (1/4 teaspoon)of salt in the centre of each weed and water in, the weed turns brown in a couple of days and you can just rub it away, this obviously isn't practical for heavily weeded lawns but is good for spot weeding, it always works for me, don't be too generous with the salt or it might samage the grass roots. Pete