Past blocked out dates for this month already ( 6/27 - 7/5 ) but if pool is determined to be at capacity hopping guests can be turned away. That being said, not that we pool hop all that much, but we usually always travel during July and August and have not ever had this happen anytime we've hopped during these two months.
Some pools are permanently blacked out - all the pools at BC/YC/BCV, BLT pool, both pools at AKV/AKL. Plus there is no hopping to the Swan and Dolphin ever.