My 2 cents :

    J'en veux = I want of it
    J'en veux plus = I want more of it (s pronounced)
    Je n'en veux pas = I don't want of it
    Je n'en veux plus = I don't want anymore of it (s non pronounced)

My 2 cents :

J'en veux = I want (some of) it

J'en veux plus = I want more of it (s pronounced)

Je n'en veux pas = I don't want (any of) it

Je n'en veux plus = I don't want anymore of it (s not pronounced)