Basically, I'm looking for a manga, where protagonist doesn't want to have his powers and seals them or looses them on his own free will.

Rurouni Kenshin he refuse to use his sword to kill
so he kind of seals his sword lol

Gintama fits on a couple of the characters...mostly gintoki and kagura

In Defense Devil the protagonist has given up his powers.

In Aiki his powers are sealed against his will, but by his father for training purposes or something iirc.

Gantz the main character wants to win the game to get back his ordinary life, which also means losing the powers the suit gives him.

Last edited by liquorice at 3:00 am, Jul 1 2010

Highly recommend Ao No Exorcist, main character seals his powers within a sword type thing,,,its really good read!

Dragon Eye main character has a lot of seals so that his power doesn't go out of control

Saiyuki - Hakkai

Chrono Crusade

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Psycho Staff it's short but it sure fits what you're looking for:
The main protagonist refuses to use his psychic abilities. Instead, he chooses to study and achieve something in life through hard work.

Megalomania
En passant
I second Gintama.

Psycho Staff is a nice one, thanks.

king of hell

Not actually that he himself sealed his own powers. But yes, they are sealed.