G-d gave Israel specific instructions on how to take the land, and what to do once they were there.  One of these instructions called for the total destruction of temples, altars etc. and the erasing of the name(s) or symbol(s) of all false g-ds worshiped by the indigeneous people.  This can be found in Deuteronomy Chapter 12, verses 2 & 3.   In  verse 4 of that same chapter, G-d makes a simple statement "You shall not do so to your G-d.".  Thisstatement is classified a "Negative Mitzvot" (thou shalt not....), It is counted as number  458 out of the 613 Mitzvot identified by Israel.

The Name of the G-d of Israel.
G-d's name consists of four letters.  These four are called, by some, Tthe Tetragamatron. Often times in Jewish literature written in Hebrew or Aramaic, these four letters are substituted with a double Yud.  Why?  Because it is easier to manage documents containing a substitute than a document containing the actual Name.

Over the centuries the custom of altering even references to G-d (such as the word G-d and L-rd) became popular.  It may seem to be unnecessary, since neither are G-d's Name.  However, many Jews (me included) alter these words by habit. It is a constant reminder of the sacredness of the Name of G-d.  When writing the word Lord as applying to a civil leader, some Jews use the "o", others write l-rd or lord.  G-d is a bit more tricky. The only time I've ever seen an orthodox Jew write out the 'o' was when referencing false gods (note the small g).  Again, this is all "family custom" amongst some Jews.

A bit more on the subject:

The words G-d and L-rd do not come up much between Jews in conversation.  We generally substitute other names. With Gentiles, we use G-d and L-rd.  The most common substitutes are Adonai (which means L-rd) and HaShem (which means in Hebrew "The Name"). "The Almighty", "The Eternal One" and "The Holy One" are less common, but still used in some circles.


When chanting Torah, the Name of G-d is pronounced 'Adonai'.  And that is not how the word should be said.  But Jews do not say the Name, nor  write out a transliteration, as is the custom of many Christians.

English translations of the Hebrew Scriputres have also adopted the use of the word "L-rd" in place of  the Tetragamatron.  Oftentimes the reader, unfamiliar with the original text,  will misunderstand the English term L-rd. They think it means always means G-d, but it can mean a civil ruler as well. The best way to know the difference is to check the Hebrew, or have a translation that makes a distinction between the Hebrew word "Adonai" and the Tetragamatron.

Shalom