How to professionally deal with sexual harassment in academia I am a first year PhD student, and there is an older (by which I mean further ahead in his PhD) student who has been rather unperturbed by me asking him to stop flirting with me. This has been going on ~6 months now after I first met him (we don’t work at the same institute but see each other reasonably regularly due to the collaborative nature of the work).

At first it seemed like he was just being friendly and we chatted a little over social media, nothing more than two friendly work colleagues. Then he started flirting, and very obviously. I told him to stop flirting and then we could just carry on having a normal conversation instead and he did. But then after a couple of weeks he started flirting again, so I again told him to cool it. He kept flirting, and I started just responding less, and flat out ignoring any flirty messages.

He didn’t seem to take the hint (unsurprisingly given that he didn’t take me outright telling him) and started messaging me repeatedly until I responded, more than 20 messages per day from him. I kept on ignoring most of them and sending some blunt messages that did not ask any follow ups when he really would not stop messaging. I have only contacted him first when it is required from a work context. 

Over the course of about 3 weeks he finally started taking the hint and only messaging me a couple of times a week, but there was still undeniable flirting and it was making me feel really uncomfortable.

I have now unfollowed him on social media, something I know I should have done much earlier but the issue is he is in a more senior position than me in the collaboration and I was afraid of making a bad working relationship by angering him.

I’m worried now what will happen when he notices I’ve cut off social media contact and I have to see him in person again soon. Additionally there is a chance that shortly I will be working more directly with him, and thinking about this possibility is making me really anxious.

I’m unsure what to do in this situation as he is not in the same institute as me. I am nervous to tell my supervisor as I don’t want this to tarnish my professional reputation, I’m at the very start of my career.
While you seem confident that the individual's behavior is unprofessional, other people in your situation might not be as confident. If you have any doubt about if someones behavior is unprofessional, find a trusted colleague and ask them in confidence.

Once you are confident that the behavior is unprofessional, the next time he emails you, send him an email that clearly states that his behavior is unprofessional and that you feel sexually harassed. Then tell him that while you have endured it in the past, that it must stop and if necessary you will report the behavior to your supervisor, his supervisor, and any other relevant offices at your and his institutions. Finally, close with saying that you hope the behavior will stop and that this can all just be water under the bridge and that you can continue to have a professional collaboration. If you have any doubts, show the email (you can write it now), to a trusted colleague and get feedback.

If, after sending the email, the behavior does not stop (or he argues), you tell your supervisor. You can either work with the supervisor to bring the fury that is the university HR department when faced with issues of sexual harassment or a gentler back channel process. The choice is yours and anyone that would hold that choice against you is not someone you need to form collaborations with.

You want to start documenting things. This means saving the harassing emails. You also want to email him so you have a copy of what was said. Do not tell him in person (both because he doesn't deserve that type of interaction and because it gives you a record of what was said). If/when you talk to colleagues for advice or to your supervisor, do that in person or on the phone. You don't want them to feel like they are being watched. If your supervisor does not react as you hope, then follow up in writing.