The pdfToolbox plug-in for Acrobat can be easily removed by deleting the pdfToolbox__.acroplugin package in the callas software subfolder in the plug-ins directory for Acrobat, under Mac.
Note: The file name contains the respective pdfToolbox version number at the "__" position.

Acrobat DC

Please note that as of Acrobat DC, Adobe has changed the folder in which it stores third-party plug-ins:
/Library/Application Support/Adobe/Acrobat/DC/Plug-ins/callas software 

Acrobat XI and older

For Acrobat XI and older, meanwhile, the plug-in can be found in the following folder:
/Applications/Adobe Acrobat XI Pro/Adobe Acrobat Pro.app/Contents/Plug-ins/callas software

On Windows, the plugin can be found in the subfolder of the plugin directory for the current Acrobat version:
Acrobat\plug_ins\callas software

The folders and files contained within can be identified by their name, e.g. “pdfToolbox 9” or “pdfToolbox 13”.

Delete them to remove the plug-in.