Hi all,
I found a solution by myself:
The lights are on, as long as openHAB holds a connection to the IRobot.
(when you use the IRobot App, it is the same case).
Disabeling the “IRobot” Thing helps, and the lights switch off a few seconds after disabeling.
Sadly, disabeling an Item is only possible via an API-call.
Thus I added this code to my rule, when the Robot should be enabled for cleaning:
sendHttpPutRequest("http://user:password@openhabianpi:8080/rest/things/irobot:roomba:192_168_178_45/enable", "application/json", 'true')
After the rule execution I do send a false value with the same HttpPutRequest and everything is fine 
Cheers
Jens