Well...
Lead (as has been mentioned), Mind Blank, and Mage's Private Sanctum stop it cold; spell resistance, increased will saves, Nondetection (also mentioned), and being on another plane (Ethereal Jaunt, Etherealness, Plane Shift, Rope Trick, Mage's Magnificient Mansion) make scrying trickier in one way or another: Scrying and it's Greater counterpart both permit SR and Will saves; Nondetection requires a caster level check before such things will work, and being on a different plane grants a +5 bonus to the defender's Will save. Detect Scrying, while it doesn't actually stop the sensor, lets you know about it so you can do something about it. There's a couple of others along similar lines - the XPH, for instance, has an actual trap power for those attempting to Scry on you.