I'm working with activities and sales documents in CRM 7.0 EhP1 WebClient.
If I try to save a document without fill the mandatory fields, the system returns a error message, but even so he allows to save the document.
How can I do to unable to save the document while these fields are not filled?
Usually, WebUI issues the error message and you must capture message and tell to abort the saving process.
In your case, you should check BADI CRM_ACTIVITY_H_BADI (for activities) or ORDER_SAVE BADI (for sales documents) and trigger your error message from there.