I don't think adding slackware64 or extra will do anything. I think it would require an actual name within the package. If you were using regular slackpkg, you can blacklist all of Eric's packages using the following (you may also want to add
[0-9]+_SBo if you use slackbuilds.org for any build scripts).
However, as Eric mentions, you can set precedence to his packages and not run into any issues with official slackware versions overwriting his packages. I'd suggest reading his blog post on the extension as he goes in-depth explaining what it is and how to properly set it up.
http://alien.slackbook.org/blog/intr...-repositories/