Avoiding stealing a resource when moving the robber in Catan
Answering the question and its follow-ups:


In general, you are not allowed to decline to take a resource. Page 12 of the rules gives a more detailed explanation of the rule Pat Ludwig cited. There is no option about whether or not to take a resource card if you place the robber next to a city/settlement of one or more players, all of whom have at least one resource card. However, as the answers to your other questions will indicate, you can may have some tactics to wiggle out of this requirement if you really want to move the robber  away but not get your (presumably 8th) resource.
Yes, you can deliberately place the robber in a spot where you will not able to steal a card. P. 12 again states, "You must move the robber away from his current spot and
onto the number token of any other terrain hex." (p. 11 clarifies that the desert is legal even though it does not contain a number token.) Any other indicates that a hex is legal even if it contains no neighbors that have a stealable resource.
Yes, you can deliberately choose to steal from a player who has no resources. Again from p. 12:


  If there are 2 or more such players, you may choose your victim.
  The player you elect to rob keeps his cards face down while
  you take 1 of his cards at random. If he has no cards, you get
  nothing!