If you ported your all the lines on your account that should close it once the porting process is complete. You can also call them once ported to go over your final bill and anything else to close out the account. This is a user supported forum and even the TMO employees on this forum don't have account access so you will need to talk to customer service. Also note that there is no proration of accounts so depending on your billing cycle you will have to pay for the remainder of the month and just know all the carriers do not prorate service.
Here is their contact info;
Manage your T-Mobile account