The playbook can install and configure synapse-simple-antispam for you.
See that project's documentation to learn what it does and why it might be useful to you. In short, it lets you fight invite-spam by automatically blocking invitiations from a list of servers specified by you (blacklisting).
Add the following configuration to your inventory/host_vars/matrix.example.com/vars.yml file (adapt to your needs):
matrix_synapse_ext_spam_checker_synapse_simple_antispam_enabled: true
matrix_synapse_ext_spam_checker_synapse_simple_antispam_config_blocked_homeservers:
- example.com
- example.net