The engine will not turn off!

This morning we found the battery dead due to the key being turned over but not started all night. We jumped the battery, it started and we took it for a drive. Again we left the key turned over and the battery died a seccond time. We jumped it and let it sit and run for about 30min. When we tried to turn off the van.. it ust kept running! The key was off and out but the engine was still running and the lights for air bag and door open and such stayed on. We were able to kill the engine by disconnecting a fuse and turned the lights on the dash off by another fuse. We also disconnected the negatibe on the battery just to be sure. I have looked around and not found a similar problem.. any advice?
Deanna