How can I destroy a block from afar?

Solution 1:

Instead of building a bridge you can also build a pile. Position yourself underneath the floating block and start placing blocks below you.

After your done destroying the floating block start digging down till you reach the ground. This way you don't have to break any glass in your tower and you don't lose any resources beside some tool durability.

Solution 2:

If you want to destroy a block from a distance, a TNT cannon could be used. This would use more resources than just building a bridge to get there as every TNT takes 4 sand and you would need about 6 TNT per shot. Right there that would be 24 glass without counting the other materials needed (gunpowder, redstone, etc.)

http://www.instructables.com/id/Minecraft-TNT-cannon/