Why was Yehoshua incensed? One who withholds his prophecy is Chayav Misah bi'Ydei Shamayim (Sanhedrin 89a)!
Rashi #2: Because they were prophesying that Moshe would die and that he, Yehoshua, would take Yisrael into Eretz Yisrael. 3
Ramban #1: He thought that they were prophesying from the Atzilus of Moshe, together with the 70 elders, in which case they ought to have prophesied beside the Ohel Mo'ed, like the 70 did, in which case they had no business to prophesy independently in the camp. 4
Seforno: Because he suspected them of deliberately withdrawing from the lottery in order to show that they could prophesy independently, without Atzilus from Moshe.
Ramban #2: This was like ruling in the presence of their Rebbe (for they were taking his expertise from him
Rosh: A hint to this is "Misnav'im"
What did Yehoshu'a mean when he said "Kela'em"?
Ramban #2 (citing Targum Yerushalmi) and Targum Yonasan: He meant that Moshe should (pray to Hashem to - Targum Yonasan) withhold 5 the spirit of prophecy from them.
See Sifsei Chachamim.
Because he thought that, since they had withdrawn and not been appointed elders, they had no right to prophesy.
Rashi: Because they were prophesying that Moshe would die and that he, Yehoshua, would take Yisrael into Eretz Yisrael. Ramban - because they were prophesying falsely and needed to be jailed, like a crazy who walks around prophesying.
Why was Yehoshua allowed to call Moshe by his name? Gechazi was punished for calling his Rebbi Elisha by his name!
Rashi (Sanhedrin 100a) permits saying 'Mori Rebbi Ploni.' Presumably, likewise one may say 'Adoni Moshe.' (PF)
QUESTIONS ON RASHI
Rashi writes that he should place on them communal responsibility to stop them from prophesying. How will this stop them?
Riva: Amidst the great toil they will be sad, and the Shechinah does not rest upon sadness or laziness, only amidst Simchah. Kela'em is an expression of ceasing.