Just to verify, from the Arduino to the relay board you're only connecting VCC and the relay IO pins (no GND)?
FYI, the diodes on the relay board are there to protect from the EMF spike caused by de-energizing the relay coil. They do nothing with respect to EMF spikes from, in your case, the fan. Hopefully someone else will comment on that.
Putting a small capacitor and/or a varistor(both appropriate rated) across the relay contact (not the coil) might help.
For the resistor, I guess I will need to find one that supports more than 25w as well.
Hi I have a similar relay operating a 12v pump my arduino resets when it shuts off.. I was hoping to learn how to work out which parts I need to use to create a snubber.. my relay has the 12v power source + the positive terminal of the pump I'm running in it.. so would I connect the snubber across those 2 lines?