Can I sue a site for accusing me of bullying based on reports with no evidence?

You asked, "Can I sue?" The answer is always yes. Simply complete the appropriate paperwork and pay the court filing fee. Winning is another matter. Many lawyers will offer a free 15-minute consultation to advise you of your chances. If not, many cities offer free legal aid clinics. Call your city councilmember's office for a referral. Good luck!