Yes.
If you want someone not to be able to call you on your 2ndLine app 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 Messages screen:
From the list of senders, swipe the row (name or number) right to left.
Tap More
Tap Block from options.
To block a number inside the app, from call history:
From the home screen, tap the burger menu icon on the top left to the dialer icon.
From the options, tap Call History
From the list on the Call History screen, swipe the number you want to block, from right to left.
You will see a red button Block, tap on the same to block.
To block a number inside the app, without any past communication (on the fly):
From the home screen, tap the burger menu icon on the top, left to the dialer icon.
From the options, tap Blocked Numbers.
Tap the Add New..., and enter a full number including country code.
Tap the Block button to block.