If you wish to close your account, go to Settings in the Blocktrade app and at the bottom you will find "Delete Account" button. 

You will need to withdraw all your funds prior to that. If the amount left on the account is less than a minimum withdrawal amount, it will be considered as dust and will be written off. There will be displayed a warning regarding this.  

After all funds are withdrawn, you can click on Delete Account and confirm the action in the email that will come to your registered email address. The confirmation link in the email is valid for 5 minutes.

Please be aware that after your account is deleted, you cannot reopen or create another account with the same email address.