in soldat server 2.6.0, OnCommand will look like this:
function OnCommand(ID: Byte; Text: string): boolean;
as you can see, OnCommand itself has a boolean result, which is either true or false. true means, that the command passes through and if it is a valid command, it will get executed. if the result is false, the command will be ignored. so you can just script something like this:
function OnCommand(ID: Byte; Text: string): boolean;
begin
If copy(text,1,6) = '/gmute' then
result:=false
else
result:=true;
end;