well... a ratchet block will not stop the line from running free, only slow it a little.
An Oxen block would, but... I think they're ~$150 and that might be over kill for a tree house. You could get a single block with cam cleat.
Something like the H231
http://www.apsltd.com/Tree/d261000/e260621.aspmatched with a triple block would give them some purchase.
Or something like a H187
http://www.apsltd.com/Tree/d261000/e260627.aspSadly though, none of these are really cheap options.
