There are a couple of things you can do, depending on whether or not you have room for another member of your team.
The first and possibly easiest choice is to consult a breeder and adopt an egg. If your swanna is indeed under the influence of her nesting instinct (and you will know if she is—the behavior you described alone is a hallmark of such for swanna, but she may also be attempting to build a nest somewhere close to your home), then allowing her to bond with an egg may be the best way to pacify her. Be sure that the first pokémon the hatchling sees is your swanna, and she will be able to channel her energies into raising her adoptive child from there. You will, of course, be expected to offer some level of care for the newborn as well (particularly in the form of food and toys), but your swanna will fortunately see to grooming, early training, and general basic care, particularly if you were able to secure a bird pokémon’s egg for her.
Alternatively, if you have no room for a new team member, you may be able to placate her with toys and distractions (such as battling), but if she’s aggressive towards you, her own trainer, then this may be less effective than the above solution. Still, it’s certainly worth trying; there is always a chance that with enough distractions, your swanna will calm down just enough for you to reason with her.
Best of luck, anonymous!