Practically, there is not much you can do against such abusers of the RTI Act. You may consider serving a notice of defamation against the information seeker and make a criminal complaint against him, for, no information seeker can make such frivilous application, which is not even "information" within the meaning of the Act but at best, "opinion" based on frivolous and whimsical allegations. Attendance and reasons for leave may not be denied to information seeker, insofar as they are a part of record related to performance of public duties. But not allegations of influencing etc; that is not information.
See Central Board of Sec.Education vs. Aditya Bandopadhyay & Ors. at 2011 (8) SCC 497 where the Supreme Court made some observations on the abuse of RTI Act and P.Jayasankar vs. Chief Secretary to Government of Tamilnadu, W.P. (C) No.3776 of 2013 before the High Court of Madras decided on 18.02.2013. In the latter case, the Madras High Court observed that the Information Commission could 'blacklist' the information seeker who was abusing the process of law -
"Commission recorded that the petitioner was abusing the process of law and his only intention is to make complaints against the Commission and he continues to make insinuating or defamatory remarks against the Commission. Therefore, the Commission passed an order that the Commission should not receive any order from the petitioner concerned ... no information seeker can be allowed to insinuate or defame the Commissioners in the guise of prosecuting their cases. 15. Under such circumstances, when specific power is vested on the Commissioner and the Commission had proceeded against the information seeker, who had abused the Chief Information Commissioner in the course of his proceedings, it will be open to the said authority to disqualify a particular information seeker by passing a speaking order".
Also, your Dad's office might be having its RTI Rules; for instance, the Delhi High Court RTI Rules advise the information seekers not to use foul or defamatory language or ask for extraneous information; check if his office has something similar in place.