It's more of a spamming filter than a limitation. However you can get around it by adding a delay to your program.
Here is a sample vbs program that I wrote to test this. ======================================================== Code Below ========================================================
Dim i, WinShell, ldate, lNumSeconds, lNumPrompts
'This delay is what allows you to send more than 6 prompts. lNumSecondsDelay = 2 lNumPrompts = 20
Set WinShell = WScript.CreateObject("WScript.Shell") ldate = now - 1 For i = 0 to lNumPrompts while not now-ldate >= lNumSecondsDelay/(24*60*60) wend lDate = Now WinShell.Run "net send ejohnson test" Next Set Winshell = Nothing