Don't demand they remove advertising. You may own the land but not the board - especially if it's a national company managing it. Demanding they remove advertising may have them remove the board in total if they own it. Contacting an attorney is sound advice.
Also you could ask for the current leasing agreement the company had with the landowner. You can then determine if the terms are agreeable to you. A national billboard management company can get the big advertiser names for higher dollars than you can locally - and you could be earning 20% of the rental fees with absolutely no work on your part and only property taxes as an expense is about the sweetest deal ever in real estate. Is the board location in your local area or is it far away? If far away, keeping the current manager may be a good choice. You make less than if you handled the advertising yourself, but if it's far away, it's a nice no-work income stream.