Well now, I got lucky with a collecting agency when I was younger.
I, like many other kids right out of highschool was sent a ton of credit card offers, but wasn't responsible with one of them.
After a few years, the collection agency sent me a letter (no doubt a typo), with me owing a balance of $0.00.
Once you have that, your debt with them is wiped clean, and you have proof that you owe them nothing.
Collection agencies will have you believe that they can do anything they want to do to get money, but the fact is, they bank on a lot of people not knowing about, or at least in full detail..the fair credit act (at least in the US).
There are a lot of things they can't do, but do anyway, thinking that the consumer doesn't know any better.
For example, they cannot tell you over the phone that they will get a judgement on you, and then do it. They have to send you a certified letter first, as a judgement is a legal proceeding. Yet many of them use it as a scare tactic, and rarely follow through, because in most cases (unless you're Bill Gates), their court costs to start the proceedings are far more expensive than the average person's debt.
Another example about agencies that call you on the phone....
They are not permitted to call you before 8AM YOUR time, and after 8PM YOUR time. Some of them use the excuse that "It's only 5PM here" etc, using their timezone, it's still in violation of the FCA.
They also cannot call your cellphone or work phone unless you've given them express permission to do so, as your cellphone costs you money, and your work phone can cost you your job.
In the end, after 7 years, your debt is considered wiped out. So if your customer just ignores them, there is absolutely nothing you can do, and ultimately do you want to pay an agency to try to collect for 7 years anyway?