Several years ago it was possible to ask Twitter to make an inactive account (6 months of inactivity) available again. All you had to do was:
- Follow @Support (Twitter Help Center)
- Send a direct message with the name of the inactive account (example: @Support Hello, the account @diginomics is inactive for a long time, could you make it available again please)
- Wait for answer
Another possibility:
register "Digital Economics" and "diginomics" in your country (trademark registration) and contact Twitter for Trademark Policy Violation
https://support.twitter.com/articles/18367#