How to change/replace ignition

Hi Folks, I need to do something, not sure what, to disable/replace my ignition/key. Not sure how to proceed.

My idiot ex-husband still has one key to my 2007 Saab wagon. He enjoys driving by the house (no driveway, we must park on the street) and hitting the unlock button, opening the trunk, setting off the alarm, etc.

Of course he denies this, but it's driving me nuts. Please don't suggest police or the like, not going there.

So my question is: what is the least expensive/most effective option for disabling the current keys?

Thanks!