Well I will tell you the harsh truth. To get rid of them you will need to pull everything out of the tank, and dip all the plants in a dip solution with tons of salt in it. Then inspect them for thier eggs and any thing that hangs one. Would do this several times. Pull the filter off the tank let it all dry out for 5 days, empty the tank and completely dry out substrate and anything for 5 days. Then put it all back in and start the cycle over.
I got leechs from some plants I bought, they infested everything hide in the substrate in the filter media, laid eggs on the under side of leaves. I had to throw away a couple hundred dollars in Xmas moss and another 200 in fissidens. had to house my shrimp in 5 gallon buckets for 3 weeks until I got the tank up and cycled again after drying everything out.
Since then i have seen none, have had no issues. Leechs are not killed easy. I soaked one in bleach for a day, pulled it out put it back in a cup of water and it went on doing its business.