"The crystalline structure of speleothems can be very porous, making them vulnerable to damage if people touch them. The natural oils in our skin will leave a light coating of oil on the formation. While one touch has only a minimal effect, many deposits of oil over time prevent any further growth. When calcium solution runs over the formation it normally leaves a layer of calcite behind. However, the human oils create a coating that the calcite cannot cling to."
Found this explanation in a New Zealand teachers guide to understanding caves. The idea is that this fundamentally alters the deposition of calcite, hence changing the accumulation which results in the speleothem growth and thus can cause it to stop growing in that particular location. That was always my understanding. I've also found a link from the SO Velebit website but there is no translation from the Croatian, only the abstract. I've emailed the author to see what I can find.
