How we can add or remove a signature along with the SMS?

You can add the signature along with your message by using the Add Signature option which will be displayed at the center-most part of the panel. 

To add your desired signature enable the option by clicking on the check-box beside the option of add signature. Similarly, if you want to remove the signature from your message just uncheck the check-box.

NOTE: Signature will affect your message credits by adding extra characters and space between SMS and signature.