Give no example before the silent round
The example you offer to get things started becomes the template, and you will get eleven variations of it. Read the question, say nothing else, and let the pause be uncomfortable for fifteen seconds.
Shuffle before you read the ideas out
Whichever idea is read first gets discussed longest and gets treated as the baseline everything else is compared against. Order of submission is not a ranking, so do not let it act like one.
Park the tangent instead of arguing it
Every workshop throws up a real but out-of-scope question. Leave Q&A open as the parking lot: it gets written down, it gets upvoted if others care, and the room gets its afternoon back.
Narrow in steps, never in one jump
Going straight from forty ideas to a decision means the decision is made by whoever summarises. Theme, then feasibility, then rank — each pass throws away something the room agreed to throw away.
Decide what 'leaving with it' means before you finish
The raw material is worthless by Friday if it only exists on a wall. Export the results or send round the recap link while everyone is still in the room, with a name against each of the three things you ranked.