Definitely. But what about contacting all of the conference's sponsors, and insinuating that they're racist if they don't denounce the conference and pull their sponsorship? Or encouraging all of the previous speakers to denounce the organizers? Both of those happened.
Probably both allowed, but... wow, some real nasty behavior.
Here's one of the early objectors citing a presenter who has openly and thoughtfully struggled with attending LambdaConf: https://twitter.com/swannodette/status/714851217512513536
Here's an early Storify story of reactions to allowing Yarvin to speak - you will find that the most vocal objectors have continued to bash and badmouth anyone associated with LambdaConf: https://storify.com/adngyipa/lambdaconf-debate-in-tweets
https://twitter.com/fatneckbeardguy/status/71358678287339520...
"@47deg Hi, are you still going to sponsor LambdaConf if they give a platform to someone who thinks blacks are suited for slavery?" - https://twitter.com/ekrubnivek/status/713501535515226113
"@andrewfnewman @adrienneleigh So, the racist organizers of lambdaconf selected his talk knowing who they would get? Smart move, clearly!" - https://twitter.com/extempore2/status/714502151884505088
"If you've spoken at LambdaConf: have you repudiated its organizers for making the choice to include bigots and exclude marginalized people?" - https://twitter.com/fatneckbeardguy/status/71280436894525030...
https://mobile.twitter.com/antifuchs/status/7128837259948236...
https://mobile.twitter.com/antifuchs/status/7128835537977057...
It's a bit ironic, since I hadn't really known about the conference, and it sounds really cool. After all, I can just not go to that speaker - whoever he is (the controversy really doesn't interest me) - and see some of the other neat talks. I think as far as conference prices go, it's not too bad, but I wasn't planning on it (and there's always the ever-larger cost of travel/lodging), so I suspect I won't see it =/