How to clear or delete variable value permanently from computer memory?

That's complicated, in many cases very complicated, or even near impossible.

The first problem is that not all variables can be changed. Strings for example are immutable: they cannot be changed once created. When you try to modify a string:

string s = "hello";
s = s + " world!";

A new string long enough for the revised version is created, and the original data is copied, then the new segment is appended to that. The original string content is not changed in any way (and unless you have kept a second reference to it) cannot even be accessed again.

So within C#, there are variables you cannot change or remove, and that means you cannot "clear all those variable from computer memory". Even if you did get the Garbage collector to dispose of them, that does not remove the content from memory, it just marks the space as "available" and when that specific piece of memory is reused the value will be at least partially overwritten. And you have no control over that at all: the memory allocator will use memory as seems appropriate to it at the time, you can;t force it to use any free segment.

And it gets worse, because Windows (like most modern OSes) uses virtual memory, which means that if it starts to "run short" of real physical memory, it "pages" some of it to disk to free it up, which means that at any point during execution your whole app memory may be written to disk and "pulled back" when your app needs it. And that you have absolutely no control over, you don't even know it is happening!

So you can't do it, you can't control it, and you may find it "just sitting there" on your hard drive a month later! (Worse, it's often possible to recover the last 5 or 6 data sets written to a magnetic media even after they have been overwritten!)