Can a single database be excluded from an AG backup?
Select databases. The keywords SYSTEM_DATABASES, USER_DATABASES,
  ALL_DATABASES, and AVAILABILITY_GROUP_DATABASES are supported. The
  hyphen character (-) is used to exclude databases, and the percent
  character (%) is used for wildcard selection. All of these operations
  can be combined by using the comma (,).


Ola Documentation

So as you list the databases to back up, it will look like this:

AVAILABILITY_GROUP_DATABASES,-%Db%...


All databases like Db will be excluded from the backup while the rest of the databases in the availability group will be backed up. Change that Db to be the one you want to not backup and adjust the % signs as applicable to your database.