ipad itunes match empty streaming storage

Hi,

When streaming music from iTunes Match to play at home (using Aiport express), I know the music is "written" on the iDevice.

Also, I know there's a way to empty this "cache" from time to time (to avoid that music is filling up the storage on the iDevice), but I've forgotten how to do it.

iPhone, as well as iPad.

Can somebody help me?

Thanx.