Try
"Lessee agrees that Lessor will have the right to review all content on the sign, and to restrict such content in a reasonable manner".
You may want to change this language based on the prevailing laws in yur state and to meet the exact conditions you want to place. I'm not a licensed attorney in your state, this is just a suggestion.
The biggest downside from this type of provision is that it is not real attractive to a buyer down the road, as it causes complications and restructions. But have I adopted such a provision on signs before (for example, where the landlord is a church)? Yes.