How can I exclude /dev/loop0 from Disk Space monitoring?

You can exclude block devices from being reported by modifying the collectd configuration for that plugin.

Specifically, create this additional config file:
/opt/wormly/collectd/etc/collectd.d/05-df.conf
The contents should be similar to this:

<Plugin "df">
IgnoreSelected true
Device "/dev/loop0"
</Plugin>

This configuration will cause the specified devices to be omitted. Alternatively you could remove the "IgnoreSelected" flag and specify only the devices of interest.

Once you have created this config file, be sure to restart collectd-wormly:

service collectd-wormly restart