striking off defense

Do you mean defense Counsel or Defendants or portion of defense in the Written statements?
 

can the sessions judge do it himself ...........

Yes he can (court within the framework of court of law)..... but he may or may not!

If  opposite party brings to the notice of the Hon Court, explicitly, about such a scenario, then based on circumstances and facts (merits) Hon Court has to decide and has give a decision accordingly citing the reasons either way, and final discretion lies with the court ( within the framework of court of equity).