you could get rid of the default tow trucks by doing a full install of GSX. So I downloaded fsdt_universal_installer and ran it.
Haven't you noticed the Universal Installer is not able to install FSX/P3D products at this time, only MSFS products ? I believe you must have seen the message when trying to Install a non-MSFS product.
This ability will eventually come in the near future, we are working on it, it's just that installations in FSX+P3D are more complex ( FSX has a method, P3D pre-4 has another method, and P3D4/5 yet another one ) so, in the foreseeable future, we'll get rid of all installers, but we are not there yet.
We had to add the install ability to MSFS first, because the MSFS product don't have a Trial always freely available, so before the Universal Installer, users had to keep a backup of the installer, since Digital River links normally expire in 30 days, so the Universal Installer solved that problem.
So, in order to get rid of default pushback trucks, you must run the FSDT normal installer, which will disable them during the installation-