When training up builders using the Roster of the Dead, wait until they start to pick up an armful of leaves, then pick them up and re-drop them. They'll gain building experience, but the ROD won't be any further towards completion as that only happens when they put down the leaves. That way the ROD is still available to use to train new villagers. You can use the same trick when clearing the rubble as well. Similarly you can train farmers using the hive or fishing; just wait until they've started back to the food bin and interrupt them and re-drop them. Note that this doesn't work the same way with the fruit trees: once a farmer picks a fruit it is immediately added to the total food; there's no actual need for them to complete the trip to the bin.