Spend 1 cat point, spend 1 point in that cat, spend 1 point in first tier of another tree Y, then 1 point in second tier of Y, unallocate the first tier point in Y. Now you wont be able to unallocate the category point.
Trying to accept the changes makes the game notice the error so you can't get the 2nd tier talent without spending the 1st tier however.
Example picture where I did the above process, triyng to unallocate the stone cat point.
