Apple - How to enable/disable OS X beta participation from Terminal?

As mentioned by SeanCLT, you/your system needs to be enrolled in one of the programs and you need to have the right utilities installed The statement about the utilities is an assumption, I have not tried it without. After that you can use the softwareupdate command to enable and disable the beta/pre-release source.

You can disable it via:

softwareupdate --clear-catalog

Enabling is a bit more tricky, because you need to choose your channel/source. The options are:

OS X Public Beta

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.12beta-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

OS X Developer Beta catalog

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.12seed-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz

OS X Gold Master catalog

sudo softwareupdate --set-catalog https://swscan.apple.com/content/catalogs/others/index-10.12-10.11-10.10-10.9-mountainlion-lion-snowleopard-leopard.merged-1.sucatalog.gz