Can I deport my fiance after we fulfill the 90 day period?

If your fiance entered US based on a Form I-129F you filed, the law seems clear. she must either marry you and get a green card based thereon or she must leave the United States. Your fiance will find out soon enough if and when s/he applies for a green card based on a relationship with another person. However, I suggest you notify CIS in writing [preferably via certified mail] that you no longer want to be responsible for this alien.

If you believe that the visa was not applied for in good faith, then you can notify the USCIS and Consulate and that will be investigated when she tries to make another application. Since she entered on a fiance visa, then she can only adjust through the sponsorship of the U.S. citizen who sponsored the fiance application.

Only Homeland Security and the US Department of Justice can deport people. If someone enters a fraudulent marriage, this can be reported to Homeland Security and they may investigate.