Currently it is not possible to clear the OpenIGTlink buffers. It might be handy for future versions that the user can clear the buffers.
In order to implement this the following components have to be adapted:
mitk::IGTLMessageQueue needs a clear method
mitk::IGTLDevice needs a clear method calling the one of the queue
QmitkIGTLDeviceSetupConnectionWidget needs a button that is connected to the clear method of the device