the best solution is to identify the plugin and file a bug report so it can be fixed. in the mean time: i don’t know what alt+select does, I assume it’s column select on OS X ? if it’s column select you can find its key bindings in the file Packages/Default/Default (OS X).sublime-keymap, search for Column Select . You can copy-paste then into your own keymap file via the menu Prefereences > Key bindings - User and that should override whatever the plugin defined.
the relevant section from the OS X keymap file is copied below
// Alt + Mouse 1 Column select
{
"button": "button1", "modifiers": "alt"],
"press_command": "drag_select",
"press_args": {"by": "columns"}
},
{
"button": "button1", "modifiers": "alt", "super"],
"press_command": "drag_select",
"press_args": {"by": "columns", "additive": true}
},
{
"button": "button1", "modifiers": "alt", "shift", "super"],
"press_command": "drag_select",
"press_args": {"by": "columns", "subtractive": true}
}