Sevy

I recommend making a backup of your modpack.jar file first off. Download OptiFine and place all of the files in the OptiFine zip file inside of modpack.jar. If your game freezes with a white screen, try a different version of OptiFine (just delete the modpack.jar file, make another copy of the original and work with that) and make sure you're using the correct version of OptiFine for your minecraft version.

If you are hosting the server (or have access to it), copy the 'worlds' folder into an earlier server setup (such as 1.0.13), delete the offending block, copy the newly saved worlds folder back to your original server setup and play. It'll require manually selecting earlier client builds via your tekkit launcher, but it should work.