A family of Microsoft word processing software products for creating web, email, and print documents.
I've been using Office 2010 since it was issued and have never been forced to upgrade by an ad from Microsoft. Though you state your purchased the software, it seems more likely that you obtained it, at least for awhile, through an Office 365 subscription, and the messages you see are nags to upgrade that subscription.
When reinstalling the software, you should enter the original product key, not an email address. That ensures that Microsoft can't connect your Office installation to any former subscription.
Please post the exact wording or a screen shot of the message(s) you see. Top post a screen shot, use the Insert image tool: