There are several products that have been tested for controlling cypress canker. The following paper maybe the most useful one to compare products.
Della Rocca, G., Di Lonardo, V., Danti, R. 2011. Newly-assessed fungicides for the control of cypress canker caused by Seiridium cardinale. Phytopathologia Mediterranea. 50 (1), pp. 65-73.
The most effective active ingredients for preventive treatments appear to be Azoxystrobin and thiophanate-methyl.
A good approach to slow down the disease may consist of:
1) Cutting down infected trees to reduce the chance of contagion. If only a few tips are infected, prune branches rather than take down the entire tree.
2) Cut down all Leyland cypress as it is a major vector of the disease.
3) Treat remaining healthy trees with products containing azoxystrobin (an active ingredient less toxic than thiophanate-methyl).