I want to allow a user to only receive emails but not send emails in exchange 2007, how can I do that?

Not sure if this is the best way to do it, but you could go to Mailbox Settings tab in the users mailbox in EMC, select Storage Quota properties, uncheck "Use mailbox database defaults", and then set "When the mailbox size exceeds the indicated amount, prohibit send at (KB):". Set the prohibit send value to 0, and it will not allow them to send any mail. Just be sure to NOT set the "Issue warning at (KB)" value to 0, or anything small, or the user will constantly be bombarded with warnings about exceeding the mailbox cap.

Maybe try removing all send as permissions?

How about a transport rule? It could be set to silently drop or reject any message from UserA.

Good call AndyD. Here's a resource afzal273: http://www.msexchange.org/tutorials/Transport-Rules-Exchange-Server-2007.html