what is the current rating of the external 24 volt transformer

2Amp fuse is actually too small for 0.5kW transformer, so it is definitely not working at the max current draw. So, what's the actual current on the secondary (24V) winding? What's the multimeter connected in series with the load show? You can try disconnecting valves and see if the load goes down. Perhaps one of the valves is stuck and is, in fact, energized all the time? What's the irrigation controller you're using? Do you have the connections diagram for your installation?

Anyhow, I would start my trying to disconnect loads one by one to get to the offending one.