Ball valves or hole drilled to break siphon

A ball valve won't do anything for you, a check valve or one way valve will though.

I assume these are returns from an open sump type setup? If they're returns from a canister filter you have nothing to worry about anyway as overflowing would be impossible.

For the sump the best siphon break is a simple hole *just* below the desired waterline, that way it doesn't spray water during normal use, but only allows a minimal amount to flow out in the event of an outage, just be sure the hole stays clean - as in not clogged.

Why not a closed-when-off solenoid? As in, power goes to 'noid, 'noid opens, water flows. Power outage occurs, 'noid closes, flow stops. Only issue here would be cost of a suitably sized 'noid. It's definitely something I'll be looking into, when I start planning my own in-wall...

Aside from this, are you using Durso/Stand-pipes? By design, these units will break a siphon once the water drops to the level of the open end of the Durso/Stand...

G'day all, yep using dursos but the returns will still suck water so a hole it is don't know how solenoids work!