ModemManager flooding console on embedded Debian 8

I am developing a Debian 8 (Jessie) based system for an embedded solution, using multistrap to build a rootfs. The system is headless, and can be accessed via a serial console meant for debugging and via SSH. I am having a problem with ModemManager in this system. It installs with no problems, but once I have it enabled and it actually starts up, it usually (not always but generally) starts flooding the debug console. The output is usually just meaningless stream of characters, but sometimes there are various AT-commands too. I know this flooding is caused by ModemManager because it can ceases once I remove ModemManager. I could live with some random flooding, but the problem is that this flooding almost always somehow makes the console non-responsive and that way prevents me from logging in. Sometimes, though very rarely, I have been able to log in despite of this flooding, check the IP of device and then log in via SSH. Usually, though, that option to work around the problem is not available as I can't even get to find out the IP given to the device by a DHCP server. I found out that this problem is due to ModemManager scanning for modem in that serial port. I also found out that there is a way to fix the problem using an udev rule. The rule that is supposed to work is like this: ATTRS{idVendor}=="0ca6" ATTRS{idProduct}=="a050", ENV{ID_MM_DEVICE_IGNORE}="1" My case is a little different because the serial port is a peripheral of the CPU i.e. not a USB serial port, so I modified the rule to this form: KERNEL=="ttyS0", ENV{ID_MM_DEVICE_IGNORE}="1" udevadm now tells me that the line is being recognized and that attribute (or whatever it is called) added to the attributes of the device. The problem is not yet solved, though. For some reason, ModemManager still keeps flooding the console and makes logging in impossible. Removing ModemManager is not an option because my application needs it.