Picasa is now overrun with Chinese spammers using the "send to a friend"-type feature. I mark every incoming Picasa share message as spam and Gmail still lets them all through...
Create a filter to send Picasa mail to a folder you then proceed to ignore.
I think you can even just send it straight to the bin with filters, although my memory may be faulty.
Which then trashes Picasa shares from any friends who may be or might in the future share photos.