I don't think you can disable firing.
For the international team, there are 2 things that make sense and have worked for me. Important note - make your manager's nationality that of the team you want to coach:
Option 1: There is an item that you can buy in the catalog to give you an invite to coach from your home nation.
Option 2: Decline all other international offers and hope for the best. I was lucky in my most recent career (was thinking of using option 1 after the season) and after a few years of getting crap offers I finally got the offer that I wanted the season after I won the BPL and got coach of the year. Not sure if it is a coincidence or not, but it happened. Definitely noticed a trend of getting higher quality offers based on the league I was managing.