CanI legally stand outside a client’s business with a sign that states: Consumers beware ABC has not paid us for services rendered?

Do not do this.

1) If there are *any* extenuating circumstances to the failure to pay--e.g. a dispute over how much is owed, or whether you performed to specifications or commercial standards, or when the money is due, etc.--could mean that by doing this, you would be committing defamation.

2) If you act in any way wrongfully while doing this--e.g. defaming as above; or physically touching anyone trying to enter or leave (which is technically assault), or blocking ingress/egress, or standing on property owned by the business (trespassing--do you know exactly where their property line ends?) you could further be committing tortious interference with commerical advantage.

Even if you do nothing actually wrong, do you want to potentially be arrested and/or sued because it's *claimed* that you did something improper? Even if you ultimately win and vindicate yourself, this can be a substantial cost and burden.

If you're owed money, sue this client--that's how you get money you're owed. While I of course recommend hiring an attorney, if you don't want to, or the amount is large enough to justify it, bring an action in small claims court, representing yourself; that would only cost you (typically) $30 - $60 and have a much better chance of getting you your money.