You could try calling them up by phone, and sending faxes. Heck, make a public service announcement, maybe that'll work
Heh, I dunno. Keep trying is all I can say.
Send them an email every week. A polite and well written one, mind. And make each of these emails different. After 2 weeks, try sending another letter. If possible, try ringing them up even (if you're willing to pay for the cost of the call that is). See if you can send a fax.
If you care for your account so much, and TNT see this, they may just unfreeze your account. You just have to hope that your letters and emails get into the right hands, and not into some yobbo 16 year old on work experience.
And if all else fails, try blackmailing them by saying you'll complain to all of their sponsors, and DO IT if they don't respond.