We don't believe the woman

  'שלחן ערוך אבן העזר קע"ח :כ

אשת איש שטוענת על איש אחד שהוא רודף אחריה והוא מכחישה אין להאמינה כדי  לייסרו אבל יגזרו עליו נדוי שלא ידבר עמה כלל ושלא ידורו בשכונה אחת ואם הוא מוחזק בעיניהם לחשוד על העריות ראוי לגעור בו בנזיפה ולאיים עליו שאם לא יתנהג כשורה יבדילוהו מביניהם לרעה וידחוהו בשתי ידים כמו שאחז"ל מלקין על לא טובה השמועה וכמו שכתב הרמב"ם בפכ"ד דסנהדרין:

a) A married woman who makes a claim on a man that he is pursuing

her and he denies the claim, she is not believed to have Bais Din punish him but they decree an ex-communication, that he will not speak with her at all, and not live in the same area as each other.

 b) and if in the eyes of the Bais Din he is suspect of having forbidden relations, it's proper to rebuke him, reprimand, and make him weary that if he does not conduct himself properly he will have to separate and we suspend him with two hands, like Chazal said we beat him on the no-good we hear, similar to what's written in Rambam, Perek 24 Hilchos Sanhedrin.

From this סעיף in שו"ע, it seems,

a) A woman's testimony without עדים is not

enough evidence to have Bais Din punish the man.

However, Bais Din does take action even though there

are no עדים.and  place a restraining order on

the man.

b) If there are rumors (no עדים) of having forbidden

relationship and Bais Din believes that it might be true

Bais Din takes action and even beats him to  stop this

relationship.