There is no explicit way to suppress this message. However, you can suppress all the message boxes if you specify also the /SUPPRESSMSGBOXES
parameter for the command line:
setup.exe /VERYSILENT /SUPPRESSMSGBOXES
If suppressing all the suppressible message boxes wouldn't fit to your requirements, you can wait for the mutex to be released (wait for your app. to terminate) by yourself in a loop with the CheckForMutexes
function in an event fired before the setup performs this check.