I owe arrears in Florida. Can child support freeze my LLC account that I use for payroll?

If they serve a writ of garnishment on the bank which that account is deposited, the bank will probaby freeze the account. Then the other owners of your company will be given an opportunity to explain to the court that the account doesn't belong to you.

If the LLC is owned entirely by you, the money in the account can be taken to satisfy the writ. Limited liability only works one way: it protects the owners of the LLC from debts of the LLC. It doesn't protect the LLC from debts of it's owner.

"608.4227 Liability of members, managing members, and managers.—
(1) Except as provided in this chapter, the members, managers, and managing members of a limited liability company are not liable, solely by reason of being a member or serving as a manager or managing member, under a judgment, decree, or order of a court, or in any other manner, for a debt, obligation, or liability of the limited liability company;
..."

How to protect a Florida LLC payroll account from being frozen by child support, if the owner owes arrears ?

If there is a court order where it is stated that your LLC payroll account must be frozen, then you will be able to do nothing. Also if it can be shown that all the income to the LLC ends up in your personal account, then the protection of an LLC will be considered moot by the court and garnishment can occur.