Block a document.

Hi everybody, If I enter a PO in the migo transaction and at the same time I try to open it with the me22n, the system shows me a message saying that the PO is in use. I want to be able to do the same in a custom transaction, for example if in the custom transaction I enter a PO, I don't want that this PO be available in other transactions. How can I do that?

Thanks and Regards