What to do if my former employer is refusing to send me my final paycheck?

1) Any employee can be paid on a salaried basis, so the same salary is received regardless of weekly hours.
2) HOWEVER, it is possible to get overtime while receiving a salary, if you are not also "exempt" from overtime. To be exempt, you must be salaried *and* you must also make a certain minimum annual amount *and* your job duties, responsibilities, and authority/discretion must meet one or more of the test for exemption. You can find these tests and the minimum salaried threshhold on the U.S. Department of Labor website--take a close look at the "administrative employee" exemption, since that is probably the more relevant for you.
If you are not exempt, then if you work more than 40 hours in a week, you get a premium payment (overtime) for all hours past 40. To figure it out, divide your weekly salary by 40: that is your base hourly equivalent. The premium is equal to 50% of that, and you get it for each hour over 40 you work that week.
3) If you were salaried, he can't withhold pay because you "owed hours." Furthermore, no employer may withhold pay without either a) employee consent (such as withholding amounts for insurance premiums) or b) as required by law (such as for tax withholding or court-ordered wage garnishment). 
d on what you write, you seem to at least have a claim against your former employer for the last paycheck; you may have an overtime claim, too, for the weeks you should have earned overtime, if you do not meet the criteria to be exempt. You should contact your state department of labor to discuss.