Restrict iPhone 3GS

[skullbang]

Is there anyway to release an universal app but prevent iPhone 3GS users from purchasing and installing it? Our game doesn't work very well in lower resolutions but run fine on iPhone 4 and above, as well as iPad 1.

Would be a shame to block all iPhone users just because of a device that maybe 5% will be using, but on the other hand I don't want to risk people paying for something that is pretty much unusable and leave bad reviews...