A few things:

First, although I realize the subject of the article is Notley, I don't for a second believe that this behaviour is unique to Alberta.  Isolating a single province only serves to increase animosity by encouraging further divison.

Second, take action. Expand funding for police to deal with online and social media threats and press charges.  Hold people accountable, not just for their actions, but for their words as well.  Would-be extremists receive police scrutiny.  Why would a domestic terrorist be any different?

Finally, speak out. There are no neutral observers in these cases. Either you support behaviour like this or you do not.  It's not funny. It's not acceptable. Posters who say things like this, encourage it or suggest it should be censured from online discourse.  Like the saying goes: if you aren't part of the solution, you're part of the problem.  Disagreeing with someone's political views does NOT justify threatening to harm them.