You can manage the Phone Numbers that you use to send text messages in the Phone Manager. Click on your name in the top right hand corner, then go to "My Profile" to open your settings. Click on "SMS Settings" on the left.

Next, click Phone Manager near the top left. (shown in image below)

The Phone Manager screen lists the numbers that you have purchased. You can see the purchase date for each number, and a total showing how many messages you've sent from each phone number. 

You can filter by Market by utilizing the Market dropdown option. 

You can flag numbers by Risk, either low, moderate, or high risk for spam. This refers to whether a phone company has flagged this number for sending spam. You can filter based on the risk setting as well. 

You can Pause a number by toggling the Status check mark next to a number (blue indicates a number is active, grey indicates it is inactive). 

If you wish to stop using a number, you can pause it for some time, and when you've stopped receiving responses for that number, you can click the Trash icon to delete the number. This will remove the number from your library of numbers. If you delete a number, you will no longer get responses for that number, so its useful to pause it for some time before removing it entirely.