What danger/code violation is oversized breakers?
Was the bozo that wired those circuits trying to burn your house down?

Assuming that the wires are 10AWG (i.e. 30A), this is a major problem indeed as the wires can overheat badly from excessive current draw before the breaker ever notices something is wrong!  Think of what a live toaster element inserted into your wall would do...

As to the Code, your issue violates 210.19(A)(1):


  (1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served. Conductors shall be sized to carry not less than the larger of
  210.19(A)(1)(a) or (b).
  
  (a) Where a branch circuit supplies continuous loads
  or any combination of continuous and noncontinuous loads,
  the minimum branch-circuit conductor size shall have an
  allowable ampacity not less than the noncontinuous load
  plus 125 percent of the continuous load.
  
  (b) The minimum branch-circuit conductor size shall
  have an allowable ampacity not less than the maximum
  load to be served after the application of any adjustment or
  correction factors.


and 240.4 (the exceptions in A-G do not apply to your case):


  240.4 Protection of Conductors. Conductors, other than
  flexible cords, flexible cables, and fixture wires, shall be
  protected against overcurrent in accordance with their ampacities specified in 310.15, unless otherwise permitted or
  required in 240.4(A) through (G).