If the board belongs to a team, the easiest way to take Admin control of it would be upgrading your team to Business Class, which would allow the Team Admin to take over any Team Boards.
If you want to try it first, you can start a free Trial of Business Class here: https://trello.com/free-trial
If the board is not on a team, or if you don't want to start a Business Class trial, you would need to work with your email service provider to recover access to your former colleague, or at least set a redirect on their email.
With that configured, you can reset their password, login with their accounts and add yourself or other users as Admins of that board.
You can add by invite folks, dont think you can eject them though unless your the owner (admin).
If it was someobody in your company, tell IT you need to access their maibox, then send PW reset, go in, move everyting to assign to you.
But, you can copy the complete board, in your space, just hit duplicate board ... change background colour so you know yoru in the NEW ... write new at start of board name, and continue with that one.
Just abandon the old board., remove yourself from it, and tell others to same and go with new in your control.
Duplicate works great, its identical, great for tempaltes or backup copies.
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