We were actually discussing this recently, over here:
New Menus in 5.4Previous versions used "pop-up" menu's, so this is sort of a new problem. There are a couple of options in that topic thread which basically amount to
1) Hide the command line and panes, so the player can't use them (player must use mouse to click on choice)
2) Create a menu that loops if the player doesn't make a valid choice (player can use keyboard)
We may still want to find a more "permanent" solution, but for now I believe these are your basic options. Additional questions should probably be posted in the other thread, just to consolidate the conversation.
EDIT: Cress makes some new suggestions that are not in the other topic. Building on his ideas, you could move the player into a "MenuRoom" without any exits or objects, where a turn script repeats the menu and the only way out of the room is to make a valid choice.