@Shelley205 That's why I said you should make mention of it in your listing description (and I'd also be sure to make it clear in a message to guests when they book or request to book). If a guest doesn't like the idea, they have the choice not to book, or to cancel their reservation within the 48 hour grace period.
It's your home and your rules- guests don't have a choice to agree to it or not. If they book, they're agreeing to it. Just like they can't bring dogs to a no pets listing or smoke in a no-smoking listing.
If they balk, just say "I'm sorry, but that's how I run my business. If it's objectionable to you, you are most welcome to look for another listing."
You're wise to go in on a regular basis, sorry it took a guest leaving it filthy to get to that point. Not only will it help you make sure the place doesn't get too bad, it should also tend to make guests a little more respectful if they know you'll be seeing how they live once a week or so.