Two questions

When prohibition is activated , can I chose a name like 'lightsworn' then all lightsworn cards of the opponent lose their effect?

How exactly do I use the union effect of monsters ?

No, you need to name the specific lightsworn name, using Prohibition. Union monsters are monsters that can equip itself onto another monsters or unequip them to special summon on the field.

No.

Declare activation of the union effect, target the monster to be equipped. When effect resolves u equip to whichever monster u targeted to be equipped.

does the union monster take up a monster space on the field or?

CZALM wrote:

does the union monster take up a monster space on the field or?

When you use the union monster to equip to the specific monster, that union monsters goes into the spell/trap zone. So no, it doesn't take up monster space. It only takes up 1 S/T zone.

But if you have full 5 monster zone, then you can't unequip that union monster and special summon onto the field.