Hi @nouahenri,
You can use a Many to Many rel between the Users collection and the Service collection! Same like Favorites , Following etc!
Check this out! : service subscribe.mp4 - Google Drive
To delete normal properties you can use the None field or use a input. : Check this post by Rozza : Press button -> update user -> make fields empty - #6 by Rozza ( I have showed this on the above video )
Thank you