Which king ordered the rebuilding in Jerusalem stopped after receiving Rehum's letter?

Answer: Artaxerxes

Rehum and Shimshai warned that a rebuilt Jerusalem would rebel; Artaxerxes ordered the work stopped, and they forcibly halted it.

“When the text of the letter from King Artaxerxes was read to Rehum, Shimshai the scribe, and their associates, they went immediately to the Jews in Jerusalem and forcibly stopped them.”— Ezra 4:23 (BSB)