All any company need do, when threatened by this typical deceitful bullshite from Microsoft, is site the case THEY ALREADY LOST in court, then slam the door in their face and ignore them to death.
The END of SCO (aka Microsoft) case:
SCO loses another round in Unix fight, must pay $2.55M to Novell
SCO loses again: jury says Novell owns UNIX SVRX copyrights
SCO/Novell suit is over, SCO loses
And so forth...
When you can't compete: Litigate.