You can stop syncing your contacts from your Account Settings.
To stop syncing your contacts:
Tap on your Profile picture in the upper right-hand corner.
From your Profile page, tap the Gear icon in the upper right-hand corner.
You’ve reached the Account Settings page.
Tap on the Sync Contacts toggle.
Tap Yes, I’m sure to confirm you’d like to stop syncing your contacts, or Keep sync on if you’d like to continue.