Clover is tough to kill compared to most weeds and the most inexpensive option, that carries some risk, is to just spray with a diluted mixture of roundup. There are also some expensive selective herbicides (Poast?) if you have plenty of dough. I control weeds in clover by mowing a couple times a season and plowing under for a corn crop every 2-4 years. The clover stores up a lot of nitrogen over the years, which helps get a good corn yield with minimal added fertilizer. I row plant the corn and only spray the rows with roundup which allows the clover to bounce back strong the following year without the need for reseeding.