This very lightweight plugin let players clear its or others inventories without a message,
so you can clear players inventories and the player won't see a message.
The only message sent by the plugin is that the inventory of the other player was cleared.
/silentclear clears the own inventory silently
/silentclear <player> clears the <players> inventory silently
sclear.self permission to clear own inventory
sclear.others permission to clear others inventory