Sorry I missed this one. If you haven't contacted the store yet call an attorney, first and he/she will let you know your options. What the employee who stopped you did (besides embarrass and humiliate you) was to open Walmart up to (possibly) a major liability.
Stopping an innocent party and accusing that party (and customer) of shoplifting means the employee didn't have all his i's dotted and t's crossed when he stopped you. Furthermore, he lied to you when he told you the incident was on tape. You called his bluff. Good for you.
If a lawyer won't take your case (they will usually do so for free, depending on what they think they could get you in compensation) , call the store and ask for a phone number for the district and regional managers. (both LP and operations) They will get the ball rolling. Tell them everything that happened and include details, such as, the store you were shopping in, the time of day, the item you were accused of stealing, what you purchased (if anything) in the store that day, and exactly what the LP said to you. I guarantee they will get the ball rolling.
Let them know how you feel. Tell them how the incident makes you feel. You'd obviously be uncomfortable returning to that store again. Let them know,
Two things the big chains like Walmart are extremely aware, are customer service and liability. The only way they can make this situation right for you, is for you to let them know about it.
They may offer you a gift card. It's your choice to accept it (you will probably have to sign off on any further liability) , or you may decide then to let an attorney look at your situation.