Where is this rule?

[DownTownTonyBrown]

Here is the quote from the NCAA rule book, Rule 2 Section 17. Glenn and GCB get it right. I don't believe it exists in any of the other rules. Under the other rules the umpire just protects the lines by saying "Don't do that."

Lines (Markings)
SECTION 17a. All lines on the playing field shall be marked with chalk or nonburning material that is not injurious to the eyes or skin. Lime or caustic material of any kind is prohibited. All lines must be two to four inches in width. The outside edge of the first and third base lines and their extensions should correspond with the outside edge of first and third base.
Note 1: Lines shall be redrawn at the discretion of the umpire. Every attempt should be made to avoid significantly delaying the game; however, if the lines can be redrawn between innings, the umpire may direct the grounds crew to do so.
Note 2: A batter may not be intentionally walked by erasing lines.
b. A player shall not intentionally remove any lines in the batter’s box or on the field.
EFFECT—A strike shall be called on the batter if the offense intentionally removes the line and a ball shall be called on the batter if the defense intentionally erases a line.

These NCAA rules (.pdf) can be found at:

http://www.ncaa.com/library/rules.html