Contact them and politely notify them the free trial period is for X days, their 2nd trial account will be disabled in 5 business days, and that you would be ha

Contact them and politely notify them the free trial period is for X days, their 2nd trial account will be disabled in 5 business days, and that you would be happy to help transition them to a paid account. There's no need to "accuse them of abusing the free trial" system, just state the facts. Worst case is they don't pay, which is exactly where you are at right now.