If you want to convince an INTP about something, you just need to tell them the reasons for what you are trying to convince them on. If your "reasons" are not logical or are not morally acceptable, you wont get your message through. I dont think its so much about not being open, naturally the openness varies from person to person, but usually INTPs listen to what you say and then evaluate whether or not what you say makes sense. If you make sense and are able to point out some mistake that the INTP made in logic, then INTPs tend to be very open and glad that you made the argument and corrected their faulty view on that issue. But its also the case that when an INTP hears a contradiction said somewhere by another person, he can evaluate in a split second if that contradiction is too strong for the argument to have any chance to being logical. Im sure many other INTPs react like me and will almost feel physical pain listening to nonsensical shit, its like being forced to continue a calculation that you know has a mistake in the beginning and the answer cannot be right due to a mistake in the beginning.
I mean its like this; The aim is to calculate the tax free prices of two items that include taxes(of 24%) and then see how many euros the difference is in tax free price.
So you need to remove the 24% tax from lets say 85 euros and 129 euros. 85 euros would be 64,6 euros tax free and 129 euros would be 98,04 euros. So the difference between in the tax free prices would be 33,64 euros.
However if you make the mistake of multiplying with 0.86 instead of 0.76 to get the tax free price, the end result will be wrong, no matter how well your logic played after that mistake. This is the kind of stuff that INTPs are able to pick up in verbal arguments aswell and see whether or not the conclusion even can be correct or not. And if you multiply with 0.86 there is no reason for you to continue and an INTP will most likely let you know that.
Im not using math problem as an example because INTPs would think in math, we think in logic and math is a form of logic(or maybe i should say that its logic using symbols).