If a magic item is targeted by dispel magic, will the item loose its properties forever. If you target the headband of intellect with the spell do you loose the 19 intelligence forever? 

Dispel Magic dispels only spells, not magical properties of items.

It has no effect; Dispell Magic states very specifically it dispels any  SPELL, not removed a permanent enchantment from an item.  If someone cast a Glyph of Warding on a chest to explode when it was opened, Dispel Magic would get rid of that distinct spell, but it doesn't "turn off" magic items.

The only spell that has any effect towards removing magic from a magic item is Antimagic Field and even then the effect is temporary.

There is no set way or spell that permanently removes magic from a magic item except for, ya know, destroying it. There is nothing stopping you or the DM coming up with something of course.