First of all don't spam, it's not nice.
Secondly here's how you do it lol.
I'll assume that you know how to create macros already. All you have to do is in the last line of your macro have it call itself.
If your macro is named spam then put this as the last line:
/macro spam;
That will call the macro and it will run in a continual loop until you stop it. Type /dump to stop all macros you have running. You can have multiple macros running at the same time but they might interfere with each other depending on what they do.
So just change spam to whatever your macro is called and it should repeat forever.
Remember, don't spam.