Unfortunately, there isn't an option to allow you to represent one guy wire as damaged or reduced.  I don't know how many guy wires you have per elevation, but I will assume 3 guy wires symmetrically spaced for the purpose of our discussion.  


Lets assume the guy wire is removed.  In this case you would have something similar to the sketch below.  Wind in the direction shown would have no resistance from the damaged level of guy wires, because guy wires can only operate in Tension and not compression.  In this scenario, I think the simplest way to simulate the broken guy wire is to completely remove the guy wires at this level.  If this were the only set of guy wires on the stack, then the stack would basically become a self supported/free standing stack.  If there are multiple guy levels, then remove the level with the damaged guy wire.  



The 2nd scenario is that the guy wire has reduced properties.  In this case I think your best option is to model all 3 guy wires with the reduced properties.  Even though 2 of the guy wires have greater area, it doesn't really help in the direction shown above since the lateral load resistance of the system is controlled by it's weakest cable.  

There is another problem in these scenarios, that is not easily modeled or considered in MecaStack, the imbalance in pretension loads with one cable reduced in size or removed entirely.  The two (2) good cables will still experience the pretension, while the damaged or broken has less or no pretension load.  This is a more complicated situation to analyze, since the stack should be out of plumb if the guy wire pretension is not balanced at each guy wire level.  For the purposes of this discussion, we have ignored that effect.   If the above simulations prove the stack is structurally sound, then we would then recommend that you set the two pretensions on the "Good" guys, and then adjust in the field the pretension on the "Damaged" guy until the stack is plumb at that level.  


I am not aware of any published specification that governs the inspection and maintenance of guy wires for stacks.  Typically on a new design, we recommend a minimum safety factor of 3 for guy wires.  I believe in this instance, you would have to determine what safety factor you have, and if it is less than 3 to make an Engineering judgement as to how much lower than 3 you are willing to accept.  That would be my approach on the guy wires, but then you may have a problem of the stack or stack components (Baseplate, breach, guy lugs, etc..) being overstressed due to the reduced lateral resistance offered by the guy wires.