I don't know you, but you might be surprised how big an issue this is; back in the '90s, I was a key employee at a regional ISPs, and the deadbeat problem was significant.
The standard answer to this problem seems to be having a lawyer draft a letter (hundreds of dollars) threatening a civil suit, and then actually taking the deadbeat to court. If you think a stern letter will get someone to cough up, this is a fine route to take. If you think it'll require an actual court case, forget it. Business civil suits can take years just to get into a courtroom; if your opponent hires their own lawyer, your odds of winning aren't awesome compared to the investment you'll have to make with your counsel; in the end, if your debtor simply goes out of business, you end up with nothing even if you win.