Mark,
Your requirement is that if a billing document is cancelled, then print out (of billing document) should not be allowed.
This is a requirement. (if condition1 is fulfilled, then proceed....Or else stop)
In the output procedure, for every output condition type, you can insert a requirement.
The requirement (A piece of code) should be if VBRK-FKSTO = blank, then proceed to process the output condition type.
To create the requirement (in other words, to write the code) you would need the help of an ABAPer.
Note: Check if the same condition type is being used for other processes, that do not have this requirement. If yes, then you might have to go for probably a new output condition type (which is applicable only for this process).