@Siobhan -
Given that Miro is designed as an infinite canvas, while useful, this feels like it would be counter to a basic tenet of the platform.
One way to “fake it out” would be to create an artificial “fence” around the area of your board and just tell folks to not scroll outside of that fenced area. There’s no way to prevent them from doing this, but if there’s nothing for them to do there and your workshop is engaging, they shouldn’t stray too often.
You could make it fun - put a few ancient map style warnings at the edges (i.e. here there be dragons) 
For a more permanent solution, I’d suggest adding a wish list topic so folks could vote on it.