No, you did not find that on an IRS website.  Try again.  Or better yet, try the Treasury Regulations:

§ 1.6013-1 Joint returns.
(a) In general.(1) A husband and wife may elect to make a joint return under section 6013(a) even though one of the spouses has no gross income or deductions. For rules for determining whether individuals occupy the status of husband and wife for purposes of filing a joint return, see paragraph (a) of § 1.6013-4. For any taxable year with respect to which a joint return has been filed, separate returns shall not be made by the spouses after the time for filing the return of either has expired.

There are exceptions.  If the spouse died and the survivor filed a joint return, the executor can change that election within a year. I did that once, where the deceased husband owed a lot of tax and the widow got a big refund based just on her own income.  

If the marriage was annulled, you can amend from married (either separate or joint) to single.