The "Send this page to somebody" link in Plone can be hidden using the ZMI under:

/plone-site/portal_actions

Find the action with Id sendto, clear the Visible? checkbox, and click Save.

Q: This makes the sendto link hidden. Can the sendto form still be access directly?

A: Yep. Appending /sendto_form works.

Update: The location is different in Plone 3. sendto is under:

/plone-site/portal_actions/document_actions/sendto