ignition problem on my 00 a4

Hi guy's i am new to the site and new to audi car's, i bought an 2000 model 1.8 petrol audi a4 a few weeks ago only got around to driving it this week on my first journey it was ok  starting out but when i got to my destination i can't turn engine off i was able to take the key out and the car was still running and all the dash light on as if i hadn't taken key out or turned it off, i have changed ignition switch and am still having the same problem car is now laying up as i can't drive it like this, have anybody ever heard of anything like this or know how to fix it all help appreciated  

Just stall it out with the clutch when you want to shut offf you engine until you get the problem fixed.