I understand what you are trying to do, Hoangsta, but I strongly recommend that you don't. Why? Many people that need to change font size do so for a very good reason. They might have a very old, small monitor, and need to fit more on the screen.
Even more important, they might not be able to see too well, and need to increase the font size so they can see it. If you prevent your users from being able to change the text, then those users will become frustrated with your site, and will leave. Is that what you want?
I strongly recommend you leave it alone -- you will gain nothing by making the font size unchangeable.