You have correctly noted that SS income is protected from creditor claims.  The same holds true even if it is in your bank account.  However, sometimes these funds are incorrectly garnished. And the fact is that there is not much that you can do in advance of a garnishment other than deposit the funds into an account not in your name or go to a check cashing establishment and have your checks cashed without putting the funds into bank account (you would need you to contact the Social Security Administration to have your check mailed directly to you and not automatically deposited into your bank account).

If your bank account is frozen and the money in it is seized, you can notify the court and sheriff that the levied funds in your checking account are Social Security funds and thus exempt.  The judge, clerk or creditor is not responsible to determine whether you have exempt or nonexempt funds in your checking account.  You are required to complete the appropriate paperwork to protect these funds.  Typically, you have to complete a one-page document and file that document with the court.  So long as this document is filed before the sheriff sends the funds to the creditor, you will be able to recoup your money.

As far as the banks are concerned, they must comply with legal process and court orders served on them.  Failure to so comply could render the bank responsible for the underlying debt.  Additionally, they are not legally obligated to determine the status of fundsheld in one of their accounts.  Since they handle thousands of bank levies per year, it would be too expensive and/or difficult to protect customers' funds while minimizing their legal exposure.

Note:  As to any non-exempt funds in the account, they can be garnished.  Try not to co-mingle your exempt funds with such other funds.