you can turn off the autoconnect, but currently you need to manualy edit the config file
goto your home dir > mission planner > config.xml
and edit
[
{
“Label”: “Mavlink default port”,
“Enabled”: true,
“Port”: 14550,
“Protocol”: 1,
“Format”: 0,
“Direction”: 0,
“ConfigString”: “”
},
change the enabled to false.