From the agent settings > Client Connectivity > Self-electing subnet services. You can uncheck the ARP option. Then push the settings to the managed devices to stop them from trying to run a discovery.
You can open Self-electing subnet services and disable each subnet under ARP and WAP. That's what I usually do to stop Unmanaged Device Discovery.
Assuming you're on 2016.x - the above is correct. You need to disable the XDD stuff in the agent setting.
If youre on 9.6 or earlier, you'll need to uninstall the XDD component on clients that have it.
Now ... note that I'm talking about *XDD* - not *UDD*.
UDD is (ALWAYS) run from the server (either as a task or "manually" by someone with the needed permissions). And it's run from the Core / console.
You could NEVER run a UDD scan and still get "unknown devices" populated if you've deployed XDD ... so you may want to check up on that. Maybe you've enabled it somewhere in your agent settings (see the above post).
The two techs (UDD and XDD) do work hand in hand.
XDD is "very basic" information wise ... hostname (if DNS has an entry) and a MAC/IP otherwise
UDD will have information on likely OS as well.