First off, this situation is not easy to handle for a dozen of reasons. Your "late buyers" may not wish a refund. In all honesty, we see no point in purchasing on a coin if the only type of motivation that keeps you going is a refund you so truly desire. They will decide on their portfolios without someone's else interruptions. 1/2 of the crowdfund wall is what the most of the buyers usually find sufficient.
By "Late buyers" I mean the people who are buying right now when the ICO is over according to your OP. They should be canceled and refunded. The %50 buy wall is going to get demolished if they aren't. Early investors will be pissed.