Uninstall

From the dashboard

Open the server and choose Uninstall. The agent picks the command up at its next check-in, usually within a minute, then stops and removes itself.

By hand

sudo systemctl disable --now secai-agent
sudo rm -f /usr/local/bin/security-agent /usr/local/bin/security-agent.prev
sudo rm -rf /etc/systemd/system/secai-agent.service /etc/systemd/system/secai-agent.service.d
sudo rm -rf /etc/securityemployee /etc/securityemployee-agent /var/lib/securityemployee
sudo rm -f /etc/logrotate.d/secai-agent /var/log/securityemployee-agent-update.log
sudo systemctl daemon-reload

What remains

Firewall rules the agent added on your behalf stay in the tool that holds them (fail2ban, CrowdSec, CSF, firewalld or iptables) and are listed in the dashboard; remove them there. Nothing else is left behind.