I have a couple ideas, but I should say that these are probably too far and so for that reason you should consider my ideas a joke and I don’t actually recommend you do them iykwim.

Find out where the famous speaker’s mother lives (or wife, kids, siblings) and get a fake phone number with that area code, say you are calling from real hospital that exists there and his mother (or whoever) was in a terrible accident, say you don’t want to discuss the situation over the phone but you can tell him her injuries were extremely serious and it’s VERY important that he comes down there, asap, he should drop everything.

Find out where the famous speaker is going the night before the event. Try to casually be in the same place and slip something in his drink that will make him sleep all the way through the next day (sorry I don’t have experience with roofies but in TV shows people seem super disoriented the next day so I figured maybe, depending on what you give him, you might be able to make him miss the whole event.

Hit the famous speaker’s car otw to the event, bonus points if he’s injured just enough to be too occupied to even call to give a heads up that he can’t make it. If he’s not injured at all you run the risk of someone coming to pick him up and barely/not at all derailing the event, but obviously if your goal is to injure him you do run a pretty big risk of going too far…

You could start a rumor around campus that the professor and/or his guest have accusations… and if it isn’t spreading fast enough then organize a protest outside of the event to get more students to protest it (organize it like get a student who will do what you say, you shouldn’t be publicly attached to the protest, obviously).

Frame the professor or the speaker for a crime in such a way that the cops will suspect them very strongly. So the evidence has to be pretty damming (it doesn’t have to stick, just damming enough that they’re picked up to be questioned/held during the day of the event).

Give the professor and/or the speaker food poising before the event!!!! This one might be the most effective because you can probably hit them BOTH and it’s not as unethical/criminal/violent as some of the other ones. Try to find out if they’re having lunch/dinner together the day before or breakfast the morning of the event. Restaurant workers are pretty cool, find one who looks like they smoke pot or do other drugs and pay them money to let you tamper with the food.

Pay off a student who’s already struggling—like maybe on the brink of dropping out—to set of the fire alarms right as the speaker is about to go on. The student won’t have too much to lose in case they get caught and you can vouch for them to try to lessen their consequences (like “hey guys, this is a student in crisis, look at his grades, I’ve been trying to be a mentor to this student because I saw he was struggling and I can tell you all that he’s going through enough personal issues right now, he doesn’t need to get into trouble for this. Please, this could be the breaking point that puts his whole life on a different, and worse, path. Please give some grace for his struggles and give him the chance to come back from this). As for the how: afaik smoking a cigarette inside should set off the alarms. Just make sure it’s in like the hallway right outside of the event or something, if it’s a big building you don’t want to risk being too far to trip the alarms in the actual event area.

Students get 1 extra credit point for each person they can get to attend your event, with no upper limit, good for multiple semesters (if they take an additional class with you or if they’ll be a future student). If you’re buddies with other professors convince them to make the same offer to their students about recruiting people to come to your event.

Definitely unethical but I mean, you picked the subreddit so