I am Almost to the Breaking Point

I have a contract with an entity, and that contract and its stipulations are constantly being taken to the limit, and lately, a little past the limit.  I came really close to canceling that contract yesterday.  Fortunately one of the parties that was coming close to violating the contract hung up on me before I could tell them to take it and shove it. 

I have been thinking long and hard about how to handle this situation, and I have just about decided to write them and email clearly reminding them of the contract and what it says, and that if they don't respect the contract, and me, then I am canceling it.  

I cannot speak to this person face to face because they are never available, and the longer I let this go on, the further they push me about not following the contract. It is causing me great stress every week.

Just thinking about putting it all in writing makes me feel better.