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Can you send the workshop URL to what you added? Assuming you have definitely removed them correctly, sounds to me like the addon created a lua file into your server to keep posting it or something..? Just my guess.

Possible ideas:

1. Has the addon actually been removed? I've witnessed the same addon creating multiple .gma files in the servers addon directory (possibly after an update)
2. Check your data folder for any weird text files, it is possible to store lua code inside a txt file and run it
3. Does this occur on any maps? Maps can be bundled with lua files, maybe you also have one of their maps in your servers content.
4. Not sure if this is possbile, but I've read somewhere, that some people worked on injecting code in the servers database (sv.db) file and run it from there, you shouldn't mess with that file tho. It might also contain confidential information so I'd suggest (as a last resort), hire someone professional and reputable from the forums, most devs here should be able to check the database for you I guess.
5. Pro tip: Download your entire server to your local pc, use any advanced text editor like notepad++ or atom and use their "directory search" feature, which allows you to search for any string of text within an entire directory (and their respective subdirectories and files). Search for parts of the advertisement text (maybe "Hammerfall") and let the program return the results, it will clearly tell you, where the text is used within any file.