how can I get my money from a buyer of my business who has defaulted on the note.

I sold my boat rental business in Bradenton to two buyers for $90,000. I agreed to hold the note for 2 years. They have defaulted on the payments twice. Now they say the have paid enough. At this writing they have paid approximately $23,000. The boats have me listed as the lien holder. They are still using my equipment to make money and will not answer any correspondence.