Your souls won't disappear from disuse, in fact they are souls so I don't even think they will get dusty :P
The souls are commonly referred to as character slots although it's not a Perfect analogy, and if you don't use them they simply go until you do.
Sparks of life simply allow you to take one of your souls and spark it into a character in game so you can play that character for up to 52 weeks of gameplay. If you want to have more than one character you will both need multiple souls and multiple sparks, one for each. That being said you can save your souls or sparks for later use, and theoretically only use your sparks of life on one soul your entire time playing CoE if you so choose as it won't disappear after use, only your spark of life does.
Lemme know if you have any other questions :P
edit: just to more directly answer your questions, yes you can reuse souls over and over again, that is their intent. You will lose your spark of life on perma-death, your soul can reincarnate into a new character.