How to (positively) validate on a missing/timeout mockoperation request?

The use case is that there is a process, that in normal circumstances sends a message to an external party. There are some special circumstances, where it must not send that message. I want to build a test case for these, when the message must not sent out. There is a TestCase, which builds up the call flow with the do-not-send circumstances, and 2 scripted mock operation. The first one should get called (and answered by soapui with a decline), then the second one is defined, but it should not get called. If it get's called, then the test should show failure. In reality, if the predefined mock operation times out, i always get a failure on the test - there is also a NullPointerException from soapui internals, and couldn't find a way to negate this implicit 'validation'. Probably there is a trick to achieve this.