How can I restrict answer options in a matrix style question

I have set up a resource allocation task in a matrix table (as below). I have four different blocks of a resource (PT sessions) to be allocated to four different people. The drop down boxes simply list the four different blocks of PT sessions (9 sessions, 7 sessions, 5 sessions and 3 sessions) - Is there a way (possibly a validation?) that I can use to ensure that the same block (for example '9 sessions') cannot be allocated to more than one person? NB. I have tried this with a 'drag and drop' task but unfortunately this doesn't present well when used on phones which most of my participants will be using? !