Remove your content from Reddit.
As a platform, Reddit is only as valuable as the content in itReddit is as valuable as the content in it. So by removing everything that you posted or commented from the platform, you're discouraging its usage. No more suckers being lulled into Reddit by your content.
If you're concerned about the potential loss of internet information, consider storing anything that you feel as "valuable content" elsewhere where other people can access. And, if you're too lazy to do it by hand (like I am!), two helpful tools are u/dessalines' Reddit-History and Power Delete Suite.
It should be obvious too, but don't produce new content for the platform.
Feed the Fediverse. (Or other alternatives.)
If you're reading this, odds are that you're already feeding lemmy.ml with some content. Good! Bring content here to the Fediverse, or to Tildes. It could be your own original content or stuff copied from Reddit (just don't advertise that site when doing it.)
Even building your own personal blog helps, as information found in Reddit becomes way less valuable if it is also found elsewhere.
If you're still using Reddit, do not install their app, and do not consent to their targetted advertisement.
I think that it is no surprise for anyone that Reddit Inc. lives off adbux (advertisement money) and data mining its own users. As such, by accessing the site through an advertisement blocker, and by refusing consent to personalised ads (check the preferences), you're already helping a bit.
Scorch the land, make it unpleasing.
Being obnoxious, mocking users, being abravise, or overall being a jerk is not illegal. You can always do it there, specially in subreddits with large numbers of subscribers.
Evading bans is also not illegal. If a moderator kicked you off, make his work extra hard by coming back and shitposting in the same subreddit.
Derail serious threads with uninteresting content. Noise makes any environment less valuable.
Raise awareness.
Reddit killed the third party apps out of greed, because they want to datamine you through an official app that sucks.
Reddit has been censoring its users since forever.
Reddit used to host the alt group site called The_Donald, and refused to take action against it for too long.
Harassment is rampant in Reddit, Reddit has the tools to prevent it (as the Aimee Challenor case shows), and yet refuses to protect its own users with them.
The Reddit administration is consistently lying to Reddit users, as it expects them to be stupid.
There are a thousand issues with Reddit. You can - and should - make people know them.
Maybe don't be a jerk though, even though it isn't illegal. That makes the world a worse place on the whole, and you would still be giving reddit more "monthly active users"