If the squares are firm you can trim them. The scalpel needs to be clean and dry or else it will pull the paste. If you notice that the scalpel is pulling the paste and damaging the shape of the squares (even though it's clean and dry), that means that the paste is too soft and it needs to dry a bit more. Trim one square at a time. If you cut a few at a time you will notice that the paste from one square is being pulled onto the next piece.
I suggest cleaning the blade of the scalpel after each cut square. Rather than making many moves up and down with the scalpel, make one smooth move (cut) downwards following the curve if the board. Hold the scalpel straight.