Welcome to the community, @Shalika Abeyrathne!
I'd advise you to check the dashboard of your NAS device and see all the actions that are performed there. You should have a backup feature that could easily be disabled. My recommendation is to check the NAS manufacturer's official website and their knowledge base articles that have tutorials on how to deactivate or activate a backup onto the NAS. You should be able to do that through the brand-specific utility you have installed on your desktop most probably (depending on the NAS model & manufacturer).
If you encounter any issues, please upload screenshots (e.g. via Imgur) so that I could hopefully guide you through the disabling process.
P.S. Don't forget that backups are very important. If you don't have a copy of your files somewhere off-site (NAS, external HDD, etc.) and it's only stored on your internal drives, you risk encountering data loss, so be careful!
Good luck!
SuperSoph_WD