I have the Modular First Person Controller downloaded from the Asset Store, but when the player jumps into a Rigidbody wall, they can just slide along it by using the movement keys, in other words it acts like they are touching the ground, which defeats the whole purpose of my parkour game. Does anyone have any fixes for this?
No useless answers like “what asset are you using?” and “can you describe your detail in more issue?” please.
How about… contact the person who published it on the asset store?
Try changing the colliders on the walls so they have zero friction.
This is an excellent way to get people to ignore your posts by they way.
I would categorize it directly under the unhelpful life strategies column, similar to posting compiler errors in the forum.
But you do you. 
If you would prefer a good full-featured start for an FPS controller, here is a super-basic starter prototype FPS based on Character Controller (BasicFPCC):
https://discussions.unity.com/t/855344
That one has run, walk, jump, slide, crouch… it’s crazy-nutty!!
In any case, here’s a great way to break down your problem in order to think about the individual parts of it, which is a fantastic way to often fix it all yourself.
How to report your problem productively in the Unity3D forums:
This is the bare minimum of information to report:
The purpose of YOU providing links is to make our job easier, while simultaneously showing us that you actually put effort into the process. If you haven’t put effort into finding the documentation, why should we bother putting effort into replying?