how do I abandon one of my islands?

[Nellie Knoller]

I wish to abandon my island Isla Volcano Delta.

[Rolig Loon]

Just stop paying for it. Linden Lab will reclaim it fairly soon.  The more graceful way is to sell it to some other resident.  You might even make back some of what you spent to buy it in the first place.  And then, of course, you could actually abandon it by clicking on the Abandon Land button in About Land.