One can disable the resize of a window by changing its border style to one that does not support
Here are two products that can edit the style of a window :
I found Window Detective to be much more reactive than Winspector, although working with both is very similar.
Here is how I rendered notepad to be unresizable.
After launching Window Detective and notepad, click the "Pick window" button of Window Detective
and bring it over the notepad window. The notepad window is highlighted, but ensure that you chose the entire notepad window rather than only a sub-window such as the edit-text control.
Click to choose the notepad window. Window Detective will then display it in its Window list.
Right-click on it and choose "Edit Window Styles" :
The following window is displayed :
Uncheck the style called "WS_THICKFRAME" and click OK.
This is it - the border of notepad can no longer be resized.
Note : If Window Detective slows-down the display, close its Messages panel.