Typically foxes are attracted to an area if there is a good food supply (mice, voles, pet food that is left out, etc.). So the first step is to make sure you don't have a grill, pet food, or accessible garbage can that might encourage the fox to hang around. If there isn't anything like that in the yard, then there are a couple of options. The longer the fox is in the area, the more comfortable it is going to be to stay there. So when you see the fox, go out and chase it off. Yell, clap your hands, bang pots together. Anything that scares the fox (without hurting it) should make it clear that it is not welcome in your yard. You may have to do this a few times to reinforce that the fox is not welcome.
The second approach is to hire a nuisance wildlife control operator to trap and remove the fox. You can find a list of NWCO at: http://web.extension.illinois.edu/wildlife/professionals.cfm
If you have more questions, please let me know.