Forbidding Spirit’s effect continues until your next turn even if Forbidding Spirit leaves the battlefield before then.

In a Two-Headed Giant game, creatures can attack your teammate and planeswalkers your teammate controls without requiring a mana payment.

Because it doesn’t modify the characteristics of creatures, Forbidding Spirit’s triggered ability can affect creatures that enter the battlefield after that ability resolves.

While Forbidding Spirit’s effect is in effect, your opponents can choose not to pay to attack with a creature that attacks “if able.” If there’s no other player or planeswalker to attack, that creature simply doesn’t attack.