There, now the conversation is permanent, and the ball is kicked right back into the original persons court. It may happen that you wont get a response, but if you arent sure what to say or youre offended by the initial contact this may be for the best.
Another approach is to ignore the message and let the match expire. This doesnt really help you in the quest to make meaningful matches and meet people, but it might help other people down the line. If someone sends out a lot of “hey” openers and gets unmatched as a result, they may reconsider their low-effort strategy and put a little more thought into their opening lines.
One thing to remember is that the other person might not be trying Bumble vs OkCupid reddit to be passive-aggressive (or just passive) they might just be having a hard time coming up with something to say.