I've never heard of any such thing where the customer keeps the set top box after fulfilling their contract. There is no contract, it's monthly and the boxes are rented.
Even if you get a new customer promo for a year or two, it's not a contract and their "free" modem is not yours to keep, and the regular set top boxes are never offered free for the new customer promos, AFAIK.
There are 5 main choices I'm aware of.
1) They rent you a traditional set top box, though they seem to be getting resistant to giving those to new customers and try to do #2 instead.
2) Rent you a Xumo streaming box, or sell you one, or you can buy it at retail for same $60, which has the Spectrum App on it. Some new customer offers include the Xumo free for some period of time, a year or two, then you rent, or buy for same $60.
3) Buy a 3rd party brand smart box or TV (or phone on the go) to run the Spectrum App. Ruku for example, or Samsung Tyzen TVs. They don't have an app for Google TV.
4) Hook up a HTPC which uses browser based Spectrum TV.
5) Cablecard box like a Tivo or other brands like Samsung had something... GX-SM530CF, but this may only be for existing cablecard renters, AFAIK they will not issue cablecards to new customers or add to an existing customer plan. Ironically some of their older set top boxes, literally have a hidden cablecard inside, hidden until you shine a flashlight in to see it.
Could they get their abandoned equipment working again? In theory yes if it's not too old, but in practice, their system may not be set up to provision something that's been MIA and could still be tied to a past account as an outstanding debt - they want a pretty penny for equipment not turned back in, far more than the equipment is worth, but I don't think you can get a reward for turning it in yourself if it was not issued to your account.
BYO cable modem is a limited option as well. Any regular account gets their Docsis 3.1 modem as a loaner for free, while a few legacy account types may have to pay to rent it monthly. In some areas you won't get anywhere near your full internet speed with a Docsis 3.0 modem (even if the modem should, in theory based on # of channels, be able to support the internet speed tier you have) and the 3.0 modems can't do symmetrical upload speeds.
In my case, they wouldn't even troubleshoot my speed issue unless I abandoned my Docsis 3.0 modem and took their free Docsis 3.1 modem. Unfortunately a significant down side of using their modem is you have no access to the logs or signal strength like you would on most cable modems bought at retail.
I wouldn't have bought that Spectrum set top box until after I called them to ask if it can be used, and first I would have asked the seller for the serial # and any other ID #'s on the sticker on it, so when calling Spectrum they can see the state of the equipment.
I have several phone #'s for Spectrum, not sure if this is the best one to use, but the others seem less appropriate: 1-800-892-4357