Dear friend:
You can use a differential amplifier using a matched array of transistors (eg, CA3018). With proper design you can get a CMRR better than 60db. Also you can provide high frequency response compensation, what is highly desirible in video systems.
Another advantage of the Diff Amp over the passive transform is perfect low frequency response, that you can't get from a transform.
Hope to help you,
NandoPG
in my practice this problem always coming from ground-loop since the cameras contains mains-supply with security-ground, and this GND is
common with coax sceen.
If common-mode noise is the problem you could also get a big ferrite ring core and wind the coax a couple of times through the ring, this gives quite good suppresion of HF common-mode noise.