You can always get a divorce, like this guy did...
[media]http://www.youtube.com/watch?v=aELp_Ax0wl4[/media]
On a more serious note.
Since divorce is not actually an option in Skyrim, you can use these console commands if you are not happy with your choice of marriage.
Kill your spouse
Enter console command resetquest RelationshipMarriage
Remove the Amulet of Mara from your inventory.
Enter console command SetStage RelationshipMarriage 10
Use console commands resurrect and setrelationshiprank player 4 on your spouse to resurrect them.
After following these steps you can get a new Amulet of Mara and find a new partner to marry.
(Since console players do not have the ability to use console commands, the only way to truly be rid of your spouse is by murdering them.)
Lots of info about spouses and marriage at this site : http://elderscrolls.wikia.com/wiki/Marriage