<a href="https://en.wikipedia.org/wiki/Atiyah%E2%80%93Singer_index_theorem">Index theorem</a> of Atiyah and Singer closed a substantial field of research in the 1960s. I knew people who were working in this field, and had to switch the field of their research
completely.

A more modern example is Louis de Branges proof of the Bieberbach conjecture. There was a large field of research, I would say a central field
in analytic functions theory, which could be called "coefficients estimates".
To be sure, it still exists, but nowadays is considered marginal.

Another commonly mentioned example is Hilbert's results in the theory of invariants. They closed the field in some sense though not forever.

It also happens sometimes that a new breakthrough does not really close the filed, but many people have to switch to another field because they are not equipped to understand the breakthrough. I do not want to give modern examples of such a sad situation, but according to Lev Pontryagin's own published recollections, he switched from topology 
to applied analysis in 1940s
because the new abstract language introduced by the French revolutionized the area. (Pontryagin was one of the most prominent topologists of his time.)