Another interpretation: When the spies entered the land, anyone who was about to say "These are the spies of Israel" was struck dumb as a stone.
Whoever Named the Spies Fell Silent as Stone
Yalkut Shimoni on Torah 251:17
דָּבָר אַחֵר כֵּיוָן שֶׁנִּכְנְסוּ מְרַגְּלִים לְאָרֶץ כָּל מִי שֶׁהָיָה אוֹמֵר אֵלּוּ מְרַגְּלֵי יִשְׂרָאֵל הָיָה דּוֹמֵם כָּאָבֶן.