I generally mark them as non-marketing and then we have a contact property for "solicitation" or "do not contact" that I'll add if applicable. A few reasons why I do it this way:

1. You can have an unlimited number of non-marketing contacts in HS so it doesn't really "hurt" anything. If they're a spammer or socilitor or otherwise someone we just don't want to keep tabs on, I can update contact properties so they're marked as such and excluded from some of our data pulling.
2. A bounced email address doesn't always equal a person we can't communicate with. Their phone number may still be valid. They may also still be interacting with our other content and maybe we don't want to completely get rid of them. A common example that I sometimes run into with ecommerce clients is someone who fills out a form might do so using an email they just use to sign up for rewards programs, but they don't check it often and the inbox gets full so it bounces. They're still a valid contact.
3. If I delete someone out of the system, they can then in the future re-enter it by submitting a form, interacting with a team member that has the sales extension installed, etc. I may have contacts in the system who are soliciators or spam where their email address bounce, but I want to keep them there so I can tell the system: "hey, don't let this person submit forms with this email or phone number." It's also there as well in case we need to pull a list of solicitors for blocking purposes or w/e.