Here's a few options you could use if you have a UniFi device in your Site Manager dashboard at unifi.ui.com that you'd like to remove:
You could remove your account from the device - another Admin on the server or the HostiFi team would need to do this for you
You could disable Remote Access under Settings > Control Plane > Console (for UniFi consoles) > Remote Access or Settings > System > Advanced > Remote Management (for HostiFi and self-hosted servers)
You could power off the device or disconnect from the internet for 48 hours, then you will have a button to remove it in unifi.ui.com
For option 3, when you go to open the device from Site Manager you'll see this view:
In the message above the "Back to Site Manager" button there's a small "click here" link to remove the device
If the device has been offline for less than 48 hours you'll have to wait before you can remove it, otherwise click Remove