The Halls of Mandos

[Kuruharan]

Who summons the elves to the Halls of Mandos, and how do they refuse the summons?

The Valar, or more specifically Mandos (I can't remember his real name at the moment. Namo maybe, but that might be wrong.)
I suppose you refuse it by being really stubborn and refusing to leave Middle earth.

Is being summoned nice, like an invitation or is it seen as a bad thing, like having to go and explain all your misdeeds?

I guess it depends on the sort of life you have led.

Can you go to the Halls of Mandos if you aren't killed or summoned?

Probably not. I don't think it's the sort of place one can just walk into, if you were in Valinor. If you were in Middle earth you could not get there at all (unless you were an elf) except by dying.