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

Judy's advice is golden, and will solve your problem, at least keeping learners from clicking an object the second time. However, it still leaves what I call a philosophical problem, that of counting/adding instead of checking/totaling. I discuss it here as an FYI for future occasions, or visitors.

Counting is when you add one to a variable each time an action occurs. As you have discovered, it is vulnerable to abuse, whether intentional or not. It also poses a problem if a learner is to make a certain number of choices, but wants to change their mind. If you are counting, allowing a change of mind can get pretty complicated. 

A much simpler, safer, and more secure method it to check the total. After each action go through and count how many choices have been made. This is where Judy's advice to use a change of state comes in. If you use a Visited state, count how many objects are in the Visited state. Even better, (if you need it) is to use the Selected state. Clicking an object with Selected state automatically toggles it between Normal and Selected, allowing for changes of mind. Then just count the objects that are in the Selected state to know how many the learner has chosen. I typically don't do the calculation until the learner clicks the object that indicates that they are finished choosing and are ready to continue.

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