There is no way to limit a coin to CPU only - if it gets popular enough SOMEONE will write a GPU miner for it.
The best you can do it make it a "hard on memory usage" coin, like Monero, or make it something like BURST where the hashes are PREcomputed then stored on a HD, and are complicated enough it's difficult to impossible to compute them "on the fly" fast enough to matter.
not 100% true, you can make so that even if gpu is possible they will not be better than cpu but on par like with some algo that soem coin use you have hodlcoin, which is one and vericoin another one i think