I like to go to Walmart and pick up some cheapo pillowcases, or mesh bags. Flake the rope into the sack, seal up the sack and bring it to the local laundromat. Toss it in there with warm or cold water, not hot and a little bit of a very mild detergent. Once they are done, grab the sacks and take them home to let them dry on a tarp or blanket or something like that in the sun or in a dry place.
Note: If you are going to go to a local laundromat, it might help to bring a mask along with you.
Washing the rope helps a little bit, but the sap won't totally come out until you start using the rope again.