Sounds like if you pay your tank lease then that should solve the problem. Since you owe them, if you unlock/unplug/whatever they have done to disable your service it WILL cost you more in the long run.
Legally they have over a barrel on this one. Just because you didn't recieve a statement from them doesn't mean you didn't owe them by the due date. For example, if I don't get a renewal notice on my truck tag, if I don't renew it in time I pay late fees, and if I get an expired tag ticket I pay a fine. In my case it was a $154 fine, $38 late fee, for a $62 tag. It's my responsibility to keep up with my monthly, yearly, whatever obligations.
Your best course of action would be to renew your lease, then after you use your 50 gallons of propane follow the lease termination procedure in your contract if you want to be rid of your current supplier.
Ken