Can you "lock" a variable?

I have created a quiz where the user selects their answer by dropping an object (frog) on one of 5 choices(plate).  I used the free form quiz to do this. my variable "TotalScore" correctly add the points for each answer when the frog is dropped on a plate.  My problem is if the learner changes her mind and drags the frog to another plate, the variable then adds the points for that second choice as well, skewing their score.  Is there a way to lock the object once the user has dropped it on a target so they can't move it to a different choice (and add the extra points)?  Of (better) is there a way to easily remove the points if the user changes their mind? Maybe I am going about this wrong and there is a more efficient way to build a custom quiz? Thanks!!

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