The lesson here is that a wall-wart is unlikely to be suitable for powering digital electronics unless
used with a proper voltage regulator, or unless it is already "fully voltage regulated" internally - the
good news is that mains USB wall warts are required to be fully regulated to 5V - this is what you
should have gone for I think.
A resistor won't solve this problem since the load is not constant - most useful devices with a microcontroller
are varying loads - a voltage regulator adjusts itself to deal with a varying load, a resistor cannot.
If it were a simple fixed load a resistor could be used.