No, they can't. It is best if the account where the SS funds are being deposited contains only deposits from SS, and not other income. This makes it easier to prevent the SS funds from being improperly garnished.
They cannot garnish the Social Security administration.
However, as has been pointed out, you will want to put the funds in a specially designated account and that account must only contain SSI funds. Ultimately, the burden is the debtor (you) to raise the exemption.