How can I restrict a hotspot to only being clicked on a single time?

It looks like you're using rectangles as the "hotspots." That's good, because rectangles can have states. 

Since you don't want the user to click a rectangle more than once, you could try this: Delete the Visited state, and add a Disabled state. Add a trigger that changes the rectangle to Disabled when it is clicked. 

By the way, you don't need a trigger to change an object to Visited.  Any object with a Visited state will automatically change to Visited when it is clicked. Here's more info about built-in states: PRIMER: Take advantage of built-in states | Articulate - Community