You can block another member from contacting you in a number of ways. First, if you have a Mingle2 mail from the person, you will be able to block the person by clicking the Block User link at the bottom of the message:

Next, if you have an open IM window with the member in question, you can block in that window. There will be a button of a circle with a line through it (i.e., the "no" symbol) at the upper right of the IM window which says "block", if you mouse over it.

You can also block a member if he or she has nudged you. To the far right of the nudge, you will find a BLOCK link. Clicking this link will block the member who sent the nudge.

Finally, you can block a member if that person has requested to be your friend. Again, to the far right of the friend request, you will find a Block link.