Yes. You can block another TextApp user (i.e. +T123 number).
If you want someone not to be able to call or message you on TextApp number, you can use the Block feature. It will block both - incoming calls and messages sent by that sender.
To block a number inside the app, from a received message on the Chats screen:
From the list of senders, tap the one you wish to block.
It will show a screen with chats with that person. On the top right of the screen, tap More
Tap Block from options.
We are working on enabling you to block a real number (i.e. Sim number).