Awhile ago, I got to a problem where I don't want users to be able to specify the font color for their posts. This is mainly because I sometimes change my website's theme from a light color to a totally dark one, thus, would render the coloring that they have possibly made, useless. So here's what I only needed to do.
Put the following code on your CSS file.
And that's it. Although this might not actually prevent the users from using the [color] code, it might give them the impression that it's not allowed and so they won't even bother to try it out.