This is such specialized vocabulary that it seems to me no established Finnish term exists. If you google "jarrusalpa", "jarrupylpyrä" or "köysijarru", you'll see that they have been used in some Finnish texts for similar devices - but they don't get many hits. "Köysisalpa", "jarrupyörä" or "köydenpidätin" would be very understandable, but they don't get any hits or are used for different equipment.

In the context, I would say "jarrusalpa" or "köysisalpa" work perhaps best. As the description of the product (on their website) also reads that the ratchet includes two carabiner clips and a rope, it would be acceptable to use a term that refers strictly only to the block itself.