It has definitely been asked before because it is on my to-do list. More generally, the ability to restrict add-ons and rebuys based on user-defined criteria (an expression or formula). For example, I have in my notes to add something like:
[ ] a player may no longer rebuy if: [ expression ]
Where "expression" is a formula, like: totalRebuys + totalAddons > 1
Which would limit any player to 2 rebuys, 2 add-ons, or 1 rebuy and 1 add-on.
And:
[ ] end rebuys if: [ expression ]
Example: totalRebuys + totalAddOns > 9
Similarly, this would end rebuys when the total of all rebuys and add-ons bought reaches 10.
Because it would have the benefit of all (or most) of the variables also available to points formulas, there are lots of things you could do with it, such as limit the number of rebuys per level, or leave them open until the prize pool reaches some designated point.
While this will require some work, it's not too large of a feature and I'd really like to get it in, but not until the port (v3.5) is complete.