Though sailing wing-and-wing is not supposed to be a catamaran thing, on a cruising catamaran the VMG dead downwind in a breeze is better and the ride smoother. Accept it. The moment the chute comes down, whether because you are single handing or because the breeze is too much, wing and wing is faster. Simply secure the boom as described, head down wind, and jibe the genoa over.
But accidental jibes are always a problem. Though I seldom rig a preventer while hand steering, I do when on autopilot, as I never know when we might jibe while I'm wool gathering in the impact zone. Locking the traveler at the end does help, but not completely, not if the main sheet is eased a bit as it should be. Monohull sailors take a line forward to a mid ships cleat. But cat sailors have more beam and can do something simpler.
Simply attach the spinnaker sheet to the boom bail. Because we seldom rig a preventer when using the spinnaker--we reach down wind with the chute--no new equipment i required. It can be rigged in moments and released instantly.
Need to pull the boom further forward? Loop the spinnaker sheet under the mid-ships cleat first. A block isn't really needed. By hauling in on both the mainsheet and the preventer, the boom can be held down, just as though you had and extended traveler or vang, keeping the boom stable and the sail off the spreaders.
See the short loop, doubled-up, hanging from the boom end? That line is used both for securing the boom when not sailing and rigging a conventional preventer, if we are going deep, when the spinnaker is up. It doesn't seem to get in the way and is always handy. One less thing to dig out and one less thing to stow.
More questions about sailing in a breeze? Read these, about balance and capsize.