There are 3 ways you can exclude accounts from Policy:
1) Create a new copy of an existing policy and apply a Perspective Filter to the policy that ensures only your required AWS accounts are evaluated. This may require a new perspective if one does not already exist. Once created, you could use this policy over the original policy.
2) Another option would be to begin copying these policies to your Sub-Organizations (these should include accounts that needs to be excluded) and then start to monitor them at the Sub-Organization level instead of the Master Org.
3) Click on the "eye" icon next to policy then click on any of the policy block names e.g. Metric Filters and Alarms.
Scroll down the page and you will see Affected resources associated with the Policy Block(Metric Filter and Alarms)
Click View ALL
Then Select the accounts you would like to exclude from the Policy.