What do I have to do to blacklist a stolen phone?

Here the link https://fizz.ca/en/faq/lost-stolen-mobile

you also need to notify Fizz

Notify Fizz immediately about the loss/theft so we can suspend your plan

To ensure that no one can fraudulently use your plan, log into your account immediately and fill in the following form: Report a lost or stolen phone/SIM.

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  Add your phone to the National Stolen Device Blacklist

  Fizz contributes to maintaining the National Stolen Device Blacklist in an effort to reduce the number of mobile device thefts. Anyone who attempts to use a device on the blacklist will be blocked from accessing networks belonging to participating Canadian wireless service providers.

  After you submit your Report a lost or stolen phone/SIM form, the IMEI number you provided will be cross-referenced against the IMEI number identified in your Fizz plan. 

  If this information does not match, will not be able to block your phone. If it does match, we will add your lost or stolen phone to the National Stolen Device Blacklist.

  Fizz is only able to add phones to the National Stolen Device Blacklist the last one that was been used by your plan on our network. If there is an inconsistency between our information and the details you have provided us, we will not be able to blacklist your phone.

  
  

Sorry to learn about this. Follow Mike recommendations.

For you, it seems too late, but for others reading the post, I suggest you to install Find My Device from Google (or any other similar tool). So you can locate your phone, lock it and remove your Google your Google account and erase the phone content remotely, to prevent to loose your personal informations. It can help to find your device if you lost it.