The BlockBreak Event is fired whenever a block is being placed. You can cancel the event to prevent the block from being placed.

You will need to cast the event in the function header as this class:
crafttweaker.event.BlockPlaceEvent
You can, of course, also import the class before and use that name then.

Event interface extensions

Link to event-interface-extensions

BlockPlace Events implement the following interfaces and are able to call all of their methods/getters/setters as well:

The following information can be retrieved from the event:

ZenGetterZenSetterType
ZenGetter
player
ZenSetter
Type
IPlayer
ZenGetter
current
ZenSetter
Type
IBlockState
ZenGetter
placedAgainst
ZenSetter
Type
IBlockState
ZenGetter
hand
ZenSetter
Type
String