There's an old account posting about a domain that they once owned, and I now own. Is there any way I can either get ownership of the account or get the old content removed so I don't have any branding issues when I start rolling out services?
sparkeddev is the old account.
If you own the domain then you should have the credentials to access the domain DNS and other stuffs of the domain and hosting. If you do not know what to do then contact with the actual domain company (goDadday, hostgator etc not sure in your case who they are) they will be able to help you to resolve your issue.
The user
sparkeddev last logged in on January 22, 2017 which is over a year ago. So, I think awaiting for him to get connected and then resolve your issue is just waste of time. In fact if for some reason if you find him/her still you will need to do the same thing either by on your own or by him/her.
PS: If you are asking for owning
sparkeddev then I do not think it's a valid question since account buying and selling should not be valid in the forum rule.