General Actions:
If you want to hide just the Last modified by... part, then edit templates/shortcuts.vm and comment out (using ##) the following lines:
If you want to hide both the version information and the shortcuts to comments, attachment and the others, then edit templates/layoutExtraVars.vm and add the following line at the end of the file (make sure it's outside the comment block delimited by #* and *#):
However, it is not recommended to modify the templates on the filesystem, since this makes upgrades harder. The recommended way of overriding a template is to create and activate a custom skin and attach the modified files to it.
You can set this $displayShortcuts variable in a document, to hide this information just for that document. To do this, add #set ($displayShortcuts = false) in a {{velocity}} macro.