There is no such thing as a server-side trade request. They are handled by the clients, and by Steam, 100%. The server is never even informed that a trade request is sent. The server is never even informed that a trade is in progress. The server is completely unrelated to the entire process. The only thing that servers are even made aware of are the items that were traded to any player on the server, AFTER the trade is already done.
Now, if you're talking about
this plugin, then yes, it would be possible to remove a gagged player from the list of players on that plugin's menu. However, that doesn't stop them from just pressing Esc, clicking Items, clicking Trading, and sending a request that way.