Can a swarm be dismissed/banished?
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Depends on the swarm.
If it's a swarm you yourself summoned with a spell, look at the duration; if it's got a (D) in the line, you can dismiss the spell as a standard action. Significantly, the Summon Swarm spell isn't dismissable this way, and stays around for 2 rounds after you stop concentrating; that's because the swarm can turn on you.
If the swarm is extraplanar, you can banish/dismiss it the same way you'd banish other extraplanar creatures, but subject to the swarm property; that is, a single-target spell doesn't affect the whole swarm, but area attacks work quite well.
If the swarm is native to this plane, you can't send it back to a different plane.
Did you have a specific swarm in mind?
Dismissal and Banish both target "one" or "one or more" creatures.
Swarms cannot be targets by spells that affects a discrete number of creatures, except for mind-affecting effects in certain circumstances.
So generally speaking, those spells do nothing to it.