Croplock (which always makes me think of kreplach) is when you destroy an enemy's croplands to put him into negative crop production. A true croplock involves destroying the warehouse or the granary so that croplands may no longer be built.
If you're not interested in farming someone, this is easy. Destroy the warehouse and croplands. Without a warehouse, they can only store 800 of each resource other than crop, so they cannot upgrade croplands above level 5 (level 6 costs 1170 clay). If you do want to farm them, destroy the granary. Without a granary, they cannot upgrade croplands above level 8 (level 9 costs 1210 crop). The main building is often destroyed, to restrict building processes.
An experienced player can get out of croplock with the help of other villages (his own or alliance friends) sending resources. First, upgrade croplands as much as possible, then upgrade the granary or warehouse as needed.