Is it possible to use an Apple Time Capsule just as a network attached backup drive and not a router?

Yes, this is possible and actually quite simple. To prevent the Time Capsule from acting as a WiFi access point or router, and use it only as a backup disk, just open the AirPort Utility app on a Mac (Applications -> Utilities -> AirPort Utility) and do the following:

Select your Time Capsule and hit Edit.
Go to the tab labeled Wireless (not Network as zhovner suggests, as putting it in bridge mode only stops it from being a router but does not stop it from being a network access point that your devices will connect to)
From the Network Mode pop-up menu, choose Off.
Click Update, and the base station will restart and load the new settings.

The result will be a Time Capsule you can still wirelessly backup your Macs to, but does not function as a router or access points, so none of your devices will connect to it.

This is the setup that I use; I have 3 eero routers and thus needed the Time Capsule to not be an access point, and these steps stopped it from being that but let me keep backing up to it as usual.

enter image description here From this wireless tab, just choose off in the Network mode pop-up and hit update.