Lock the block element when you are finished with it, then right click it to get the option to "open in code editor", select the "[BlockName]BlockEntity.java" and near the middle should be "GetMaxStackSize" and change 64 to 1. However I will state if you use the blockstate plugin on the block to give it more blockstates this will prevent you from running the game as you will receive an error.
Lock the block element when you are finished with it, then right click it to get the option to "open in code editor", select the "[BlockName]BlockEntity.java" and near the middle should be "GetMaxStackSize" and change 64 to 1. However I will state if you use the blockstate plugin on the block to give it more blockstates this will prevent you from running the game as you will receive an error.