There are a number of ways to contact CUPE to tell them what you think of their boycott. 
Here is contact info for the National office, for a committee that ought to oppose boycotts, and for the Ontario division that initiated this boycott.   
 
The national CUPE address is:
 
Mr. Paul Moist, National President
Canadian Union of Public Employees
1375 St. Laurent
Ottawa ON K1G 0Z7
By phone:
(613) 237-1590
By fax:
(613) 237-5508
To comment by email:
        http://cupe.ca/BE_feedback.php
 
 
CUPE has a committee devoted to eliminating discrimination
Seems that the new boycott violates the philosophy of this committee. They can be contacted at equality@cupe.ca
This is what their website states as their mission:

National Working Committee on Racism, Discrimination and Employment Equity (National Rainbow Committee)

http://cupe.ca/updir/images/national-rainbow-committee-0.jpg

May 22, 2003 02:01 PM The National Rainbow Committee is a working committee on racism, discrimination and employment equity. They review, evaluate, monitor and assist in developing CUPE policies and programs to ensure our commitment to promoting and defending the rights of all workers of colour in our communities, locally, nationally and internationally

 
  
On a Regional level, the Ontario chapter has the following leadership:
 
  • Patrick (Sid) Ryan - President Ontario Division
  • Candace Rennick - VP
  • Michael Hurley - VP
  • Wyman MacKinnon