First of all, apologies if this feature already exists and I just wasn't able to find it.

I've been here for a few months now, and I've had almost entirely positive experiences. That said, there are just a couple of users I know I'm not going to want to trade with in the future. Could a way be added to block a particular user, so that you no longer see their name in Trading Opportunities and vice versa?

No matter how much Bob I-Only-Trade-At-50%-Profit-To-Me, Joe Upgrade-My-Card-Condition, or Steve Steal-Your-Stuff might have cards I want, I'd rather french-kiss a wood chipper than give them the satisfaction.

This does exist! If you click on a users' profile, there is an option on the left-hand side to block the user (at least in the full browser mode, not sure about mobile).

If you block a user, they cannot communicate or open trades with you, and will not show up in your trade searches.

It seems to only be available to premium-tier users.