Right now, if a block can't be harvested, you have no possibility to get rid of blocks. This is fine, in general, but sometimes you want to focus on building and just get rid of the blocks in your way. Possible solutions:

• A hulk mode, which allows you to destroy blocks just like creative mode. However, this might not be a good idea in all cases. One could imagine blocks, which you shouldn't be able to mine. Think of bedrock and the mazes in the twilight forest.
• Create a special tool, which can harvest blocks, but you wouldn't get any drops. Unfortunately, this has the same issue as the hulk mode but could be easier balanced for mod pack devs.

Fixed in 1.5.1