I begin to suspect going nuclear is the only option.
"Hi, I'd like to cancel my service"
"But what about X?"
"Ombudsman and or Lawyers. Cancel. You have been instructed, the next time I say this will be in court/writing/etc".
Legal department are not lawyers, and are also usually much much smaller in size than customer support as a whole. Also legal departments are typically fairly terrible at resolving disputes in a reasonable amount of time (before months have passed). Even if your issue is simple or a misunderstanding, if you get transferred to legal, it is no longer simple at that point.
-Hi, I'd like to cancel.
-Cool, we are upgrading your account so you don't need to. -Umm, just cancel it please.
-Today only, you get 5GB for free
-I don't care, I want to cancel.
-Why?
-Because I don't need your service anymore.
-But what about the free data!?
-I don't need you, I am moving country.
-What country and when? Do you want another month before you leave?
-I am leaving tomorrow.
-But what about the free 5gb?
-If I say yes to the free 5gb, then what happens?
-Well we can't make more than 1 change to your account in a day, so you can't cancel if you accept our upgrade.
-Then just cancel.
-Ok putting your through to the right department
line goes dead
Next attempt, they take my number in case line goes dead, it does, they ring back, after one ring, line goes dead.
Third attempt, they cancel it.