A master magician dos′nt need open very easy, easy, normal, hard, very hard lock spells.
Very hard will do. You needed all those spells at some point of the game. Now they are just stockpiling in the spell list.
Whit a paper, quill and inkwell you could write down, as a scroll, that now useless spell to you.
then it′s out of your memory. Put it in a chest, sell it, or if you wish, read it, and the spell is again at your list

Might I point you to http://www.gamesas.com/index.php?/topic/1104003-tes-v-ideas-and-suggestions-171/
Good thread for small suggestions like this.

Yep, that is where this can go.

But yes, I did miss being able to delete spells easily in Oblivion.