How to inhibit Mn2+ autocatalysis in the oxalate-permanganate reaction?
At first the reaction is slow. But when we heat up, the reaction gets catalyzed due to formation of $\ce{Mn^2+}$ ions. It happens due to the fact that $\ce{Mn^2+}$ gets oxidised to $\ce{Mn^3+}$ ions. (Source).

$$\ce{4Mn^2+ + MnO4- + 8H+ -&gt; 5Mn^3+ + 4H2O}$$

So, we have to inhibit the formation of $\ce{Mn^3+}$ ions so as to inhibit the autocatalysis reaction. For this,  we have to use  phosphonate and carboxylate based chelating agent that solubilize (hydr)oxide-bound $\ce{Mn(III)}$ . The following is some relevant information from the site:


  phosphonate-based chelators that solubilize (hydr)oxide-bound
  Mn(III) via ligand-promoted and/or reductive dissolution at
  circum-neutral pHs.
  
  pyrophosphoric acid (PP), methylenediphosphonic acid (MDP), and
  phosphonoacetic acid (PAA), ligand-promoted dissolution is
  predominant: from pH 6−8, initial dissolution rates and plateau
  concentrations for dissolved Mn(III) decrease in the order PP > MDP >
  PAA, and at pH 5, MDP reacts equally well (with birnessite) or more
  efficiently (with manganite) than PP, and PAA remains the least
  reactive chelator. For manganite reacting with an excess concentration
  of aminophosphonate/carboxylate-based chelators, the
  aminophosphonate-containing iminodimethylenephosphonic acid and
  glyphosate yield appreciable amounts of dissolved Mn(III), but the
  aminocarboxylate-based methyliminodiacetic acid yields solely
  dissolved Mn(II) via Mn(III) reduction.