You can let them know that you're reaching that limit, and ask how you should proceed. If you have prioritized your work (which is your right), you can also let them know. Here is a sample of what you can say:
Hello [Associate Dean],
As you know, in response to the imposition of terms and conditions by the CEC and the colleges, CAAT-A members are now engaged in a legal work-to-rule labour action. Under this action, all faculty members are to work to the letter of their job description or contract.
According to the CEC, the total workload hours per week for a [X]-hour partial-load contract is [Y] hours, and I am quickly approaching this number. I have prioritized my workload to ensure the most critical tasks are completed first. The remaining tasks for this week include: [XYZ]. Please advise on where I should direct this remaining work.
Thank you.