Concentration. This effect lasts as long as the caster maintains Focus. The recipient of a Concentration spell that requires physical requirements (such as arms crossed or in the air) may choose to decline the spell simply by not applying the physical requirements. All normal spell defenses must be used first.
For example: Vorin throws a Desecrate spell at Terin. Terin’s Spell Shield goes off. Vorin angrily throws another one at him. This one is effective, but Terin chooses not to have it affect him simply by refusing to cross his arms over his chest as required by the spell.
In no cases can these spells be used to force an unwilling target to break another spells’ physical requirements.
For example: Terin has a Repel active against Vorin. He holds his hand before him as required by the spell. Vorin throws a Desecrate at Terin, knowing he has no Spell Shield, in an attempt to disrupt the Repel. Terin merely chooses not to accept the Desecrate, and his Repel is still active.