Official rule text
14.2 When a player blockades another player’s space dock, if the blockaded player has captured any of the blockading player’s units, those units are returned to the blockading player’s reinforcements. a While a player is blockading another player, the blockaded player cannot capture any of the blockading player’s units. RELATED TOPICS: Producing Units, Ships
View official source
Source: Living Rules Reference and FAQ - Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 14.2