Delete messages in service broker queue

I would use end conversation (that will also remove all related messages from all queues) using statement:

End Conversation @c With CleanUp

if you just receive message, then you leave conversation open. End Conversation With CleanUp is for specific situations only.

Something like this should work:

while(1=1)
begin
    waitfor (
        receive top(1)
        conversation_group_id
        from dbo.yourQueue
    ), timeout 1000;

    if (@@rowcount = 0)
        break;
end

Just combining the two previous answers (by Ben and Jānis) for clarity. This worked for me:

declare @c uniqueidentifier
while(1=1)
begin
    select top 1 @c = conversation_handle from dbo.queuename
    if (@@ROWCOUNT = 0)
    break
    end conversation @c with cleanup
end