my advice (as I worked in retail for a LOOOONNNNGGGG time)
go in and POLITELY (as hard as that may be) to see the store manager, if he/she isnt available be sure to ask for the manager on duty. (make sure to go in on a saturday/busy day so you dont get hawked or picked out again and treated roughly).
if they ask you what it is in reference to - politely tell them no. They do not have any reason to know why it is you wish to speak to the manager. once the manager arrives, tell your story.
Writing letters in my experience gets no where unless its written to district/corporate offices. Also try to find the # to the district manager, better yet, call another store in the area to get the info if the one you were @ was uncooperative.
Make sure you explain how your phonecalls went, if youre lucky they were recorded.
DO NOT:
Go in and steal (although I dont think youd do this

) as a way to get back
or
Go in and cause a scene, youll only make it worse and make yourself look like an asshole. The idea is to make them feel like shitheads and the only way to do so is to keep your cool.
If they suspect a theft, I dont blame them for watching (regardless of how right/wrong it is) but was uncalled for for them to escalate the situation in person - but what was the guy thinking (was he afraid or did you yell? I dont know which is why Im askin... if he felt threatened maybe)? also, from the sounds of it, the phone call was EXTREMELY uncalled for and rediculous. There is no store in existence where you are not authorized to give a first name, district info, or atleast get a manager (that I know of). Sounds like whoever was in the wrong didnt want to be accused of something or get in trouble.
thats my 2 cents..