MacLennan urged clubs to reject a formal request from Rangers, Hearts and Stranraer for an independent investigation into a controversial vote which ended the season for the three lower divisions and handed the SPFL board the authority to do the same for the Premiership.
Rangers previously alleged clubs had been "bullied", claimed they had "alarming" evidence and called for SPFL chief executive Neil Doncaster and legal advisor Rod McKenzie to be suspended.
They later promised to divulge their dossier to clubs "well in advance" of a general meeting called for 12 May.
In a lengthy open letter, MacLennan wrote: "If Rangers or any other club genuinely believes that it has been bullied by any member of the SPFL team, it has a duty to report that to me, as chairman of the board of the SPFL. I will then investigate any such allegations fully and thoroughly".