Preventing interpolation between 3 data points in a contourf plot?

I'm taking a transport class and I'm working on a modeling heat flow in a system via a finite difference method and then plotting the results in a contour plot. I've worked through the plot to generate the temperatures at the nodes of the system, but the system is not rectangular. It has a corner cut out as seen here:

I'm having difficulty making this display with a blank square in that region. The closest I've come is assigning values of NaN to where nodes 22-24 would occur, but contourf still interpolates data in the triangle made up of nodes 15, 16, and 21, as seen here:

Does anyone have any suggestions for removing that triangle and/or a better method of generating the blank rectangle in the bottom right? My professor says it is ok to leave as is, but I'm curious now and want to try to figure out how to solve the problem.

I've included the matrix that generates the contour below:

P = [ 1525.954138125030   1525.279445653040   1523.596090745226   1521.935746740187   1520.830668474906   1520.450690257122;
      1519.666996122022   1518.794954697074   1516.528428923648   1514.535543739916   1513.301345190155   1512.888739649621;
      1515.123936968909   1513.704948089585   1509.187126512376   1506.376654105672   1504.950428896174   1504.501577961054;
      1513.418855574446   1511.713774179982   1506.026344966314                 NaN                 NaN                 NaN]

For reference, the block is occupied by a cooling chamber with a gas flowing through at 400 K, but if I just assign values of 400K to those three nodes, there isn't enough resolution because the contour map then ranges from 400 to 1525, instead of 1504 to 1525. It would be fine to also assign it a value of 400 and then readjust the increments of the contours, but I haven't been able to figure that out either.