If you have big brother heres are 2 ways you can find out who modified a block(s)
1.
/bb here [radius] - lists everyone who modified blocks in the radius specified.
/bb here [user] radius] - lists everything [user] has done in the radius
2.
/bb log - enables the log block tool, The tool is set the the normal log by default
Place the block where the block used to be and it will list modifications in that 1
block area.
OR
Hit the block where it was modified and it will list modifications where the block
you hit.
Better documentation of the commands:
https://github.com/tkelly910/BigBrother/wiki/Commands