The focus-stealing behavior of Cinnamon seems to be a matter of some debate. See the Cinnamon bug tracker:
At the moment there does not seem to be a solution in vanilla Cinnamon. :)
However, you may want to use the hack proposed by user ac000 in issue #1727:
Comment out the line
in the function
_onWindowDemandsAttention : function(display, window)
in the file
/usr/share/cinnamon/js/ui/windowAttentionHandler.js. Note however, that this will prevent any windows from stealing focus when they require your attention. Also, this file will get overwritten each time you upgrade Cinnamon, so you may have to re-apply the hack every so often.