Placed Monster Spawners PS blocks now spawn monsters/animals.
Can this be disabled?

You mean make spawners not spawn things if they are used as pstones?

Yes, to make spawners not spawn things IF they are an active precious stone block. There aren't many blocks that players don't normally get through play, so 52 is currently my server's "test" block. As such it is small, too small for prevent-mob-spawn/prevent-animal-spawn to be effective.

I thought this used to be the default behaviour, but that may have been some other plugin interfering.

Edit: Started with 'No' instead of 'Yes'. Note to self: Multitasking = bad.

Additionally, I want to use TNT as my Cannon block, but am currently figuring out the best way to disable the ignition of said TNT block. This would fall into this issue, I suppose.

I'm not sure bukkit supports cancelling of the transformation of TNT into ignited TNT in all circumstances. This may have changed recently.

Haven't found a way to do this through the bukkit api. :(