When you see a spike at 50 Hz or 60 Hz (depending where you live), it is most likely indicative of a poor connection between the electrode and scalp, though it can also be an electrode problem.
The first thing to rule out is some kind of electrical interference close to the Q-wiz.
Unplug the AC adaptor on your computer (if you have a battery) from the wall and see what happens to the spike. Note that just unplugging the adaptor from the computer won't have the desired effect, since it is the transformer (the box in the line between computer and outlet) that makes the large electrical field.
Try placing the Q-wiz in the client's lap or use velcro to connect it to the back of the client chair--with the client between the computer, monitor, etc. and the Q-wiz.
Try using a GFCI outlet.
If these don't work, try improving the connections starting with the site showing the largest artifact.
If using a cap, put a bit more gel into the hole and lightly wiggle the electrode down against the scalp to see if the 50 Hz is reduced.
If using electrodes, remove the electrode and re-prep then replace it. Wiping your finger around the outside edge of the blue Neurotrode disk to spread any extra paste against the scalp to create a seal can often help
If using the cap: scrub the ear lobes (if they are references) with some Nu-prep for about 10 seconds before putting the cap ear clips on).
If using electrodes: make sure you scrub extra well on the placements with Nu-Prep before putting electrodes on, or using the blue disk electrodes on all sites even the ones you would normally use adhesive electrodes on can help. Make sure there is no hair under the electrodes.
If these things don't work, it may be the electrodes themselves.
If you don't clean your Electro-Cap properly between uses, that can result in a poor connection.
If you've tried all of these, and you still have some 50 Hz, look at the EEG from 40Hz down. If the signal looks pretty normal in the spectrum analyzer display, then don't worry about the 50 Hz.