The sync "knows" only the series configured and the files on the server. He only recognizes the changes that have been done to those files and the folders where the files are located.
Usually, there is no need to remove any content "outside" of it's series.

I understand the need to remove the original content in a (controled) commercial environment, but this is not what the sync should do in non-commercial environments. At least not by just enabling an option somewhere in a series. Usually, the sync is installed on normal user computers which are all different and have various content installed and so on. Wiping their installation is not what we had in mind when creating SimSync PRO.

But there is an option to remove content by using "Scan & Delete" and/or dummy files. And doing this by following this instructions is something you are doing on your own risk! We will not take over any responsibility for any data that has been lost by allowing SimSync PRO to delete files!
So PLEASE, before releasing a series with enable delete option to the "public", secure it with a password (tp prevent that somebody accidentally syncs this series) and test what the sync will do during syncing by enableing the PREVIEW option in SimSync PRO. Once you are as sure as possible what will be deleted or not, remove the password or forward the password to your drivers.

To delete data outside of the series, you need to know how the Sync "sees" the "world": He only knows the files (and only the files, not the directories) which have been uploaded to the series folder on the server.
Files which are on the local computer in the same directory (or a sub-directory) as a file on the server will be recognized and the sync will check if they have a counterpart on the server. If not, the sync will try to delete the file (if he is allowed too).

e.g. if you have a file that will be synced to the folder "content\track_a" on the local computer, the sync will recognize all files in this folder and below (e.g. content\track_a\somefolder\file.ext) and will checks if this file belongs to the series or not.
Files in folder "content\track_b" will be unrecognized, if no files are on the server in this folder.
There are 2 ways to force the sync to also scan this folder tree:
a) add this folder to the "Scan & Delete" setting of the series
b) create this folder on the server and put an empty dummy file into that folder. The filename doesn't matter but i would propose to name it dummy.txt, so everybody knows that this file isn't really used.

With this 2 solutions, the sync now is instructed about locations he should lookup for "unknown" files and he will try do delete them during syncing.

Please be aware that the setting "Remove unused content" must be enabled and "Protect original game files" must be deactivated.

A good idea is also to protect the default folders in the "Undeletable" setting. These should usually not be deleted, even not when removing the original content.

I'm not sure, if there are any configuration files in some of the original content folders which should not be deleted. If there are some, you should put those files into the "Local configuration files to protect" setting.

You may also need to define the folders in "Delete subfolders" where the sync should be allowed to delete empty subfolders from. Here you have to play around a bit with the settings.

Like written somewhere above, this procedure should work in a controlled commercial environment where all computers are more or less similar. But even here, you should test this and play a bit around to get the best results.
In a non-commercial environment with user computers, this should be avoided.