Can I just say I'm glad that this review bombing is causing so much discomfort to the fanboys? It's the most tangible outcome that we'll realistically see and it's pretty fucking great.
Where have you seen that, any links?
Yeah, the problem with just voting with your wallet has always been that it doesn't send a specific message. They're not psychic, and the much more likely conclusion they'd draw from lower PC sales is interest in PC versions declining. User reviews will at least inform them if they want to hear their audience out. It could go nowhere, but it's probably the best and most convenient option of protest for most fans, and it's worth a shot.
As someone who doesn't have GTA5 on Steam yet, I'm not sure what to do anymore. Buy it and give them money they don't deserve? Or pass and miss out on something I'm pretty sure I'll enjoy?
Voting with your wallet while giving critique in their forums, through twitter, facebook, email etc. It can work, just look at Xbox One for example. People were vocal and Microsoft backpedaled. Steam reviews are quite insufficient since it requires that you already actually bought the game, you can't vote with your wallet and raise your concern anymore. To participate in this protest you need to buy the game. Use more direct means to communicate that you aren't accepting their behaviour, hence not supporting them. The link with the lost sales coming from the modding restrictions isn't any clearer if the complaint comes from Steam reviews rather than forums/twitter etc. It's not like they were expecting those people writing reviews to buy another copy to their account.
To your question, I'd boycott Rockstar for this and rather invest in a smaller dev(s) that have made an enjoyable game(s) you are interested in. =P