I fear this is an XY problem, but I've got to start somewhere. Can the IRS place a levy on whatever they want including money due to me from customers without any notice of an intent to levy? I had received a letter from the IRS stating that I had until 6/12/2016 to respond to a 940/941 that wasn't filed and then I got a whole packet of accounts they levied on 05/18/2016. If this question is not suitable for SE I apologize. Not sure where to turn right now. The issue is compounded by the fact that the agent isn't returning my calls and the local office he is supposed to be from is indicated as being closed on the irs.gov website as well as a recorded message when calling the number for the office stating the same.
As I understand it, they are required to give notice of an intent to levy. Publication 594 gives their version of what they can do, and how. If you don't want to pay what they claim you owe, you can request a Collection Due Process hearing which means Form 12153. You may receive a CP504 Notice, which seems to be one of two "final notices": it is actually final w.r.t. levying state tax refunds, and is penultimate otherwise. Their page on levying mentions a crucial last stem of sending you a Final Notice of Intent to Levy, which is required according to IRC sections 6330 and 6331 which derives from 26 USC 6330 (most of the notice requirement) and 26 USC 6331. If you get notice CP90 which I think is called Final Notice of Intent to Levy and Notice of Your Right to a Hearing, then, dude, you've been served. There is in fact additional process that they have to do before actually taking, but that as far as I know, is the last part involving notification. Take note of the US Code parts, 26 USC 6330, 6331, because unlike IRS advertising, the US Code states what the law is.
The IRS can do pretty much whatever it wants. Sometimes, with appeals to the government, you can get it to stop or undo something that is or was particularly egregious. The easiest and most direct appeal you can make, other than to the agents handling your case, would be to the "Taxpayer Advocate Service".
(If the IRS did something really bad and newsworthy you might be able to get the attention of a major news outlet, or even a Congressman, to put pressure on the agency. If you have a lot of money, or can generate a lot of sympathy from a law firm, you can appeal to the judiciary for relief.)