I have logged this request but honestly I don't expect to get to it any time soon.
Even if we enforce this at the cart nothing prevents a user from making multiple purchase transactions. Even if we were to keep track of previous orders someone could use a different user account. So to really enforce it is not trivial and 100% enforcement is not feasible.
WebStore is a very basic ecommerce feature and I don't expect to evolve it much until after I complete more of my planned add on products. I built it to support my needs and I share it as it is in case others may find it useful. I do plan to evolve it but its a lower priority vs other things I need to complete to keep my business model viable. Possibly I may try to figure out a way to plug in additional functionality so that I can sell the advanced functionality as add on extensions for WebStore.
I also don't want to make it compete against my other products, ie I think Event Calendar Pro is a closer match for a race entry scenario though it may not have everything you need. But it would be bad for my business model to make improvements to WebStore to support event registration kinds of ecommerce scenarios. Ultimately I have to make a living from my work if I'm to continue doing the work, so I can't do everything as free features.
Best,
Joe