Evicting an RV on a month to month lease

We live in Parker County Texas. We rented an RV space to a couple on our property. They violated the lease by having someone else stay with them that wasn’t allowed on the lease among other things. They pay $500 a month in rent due on the 3rd of the month, per a verbal agreement, the lease says the 1st. They paid rent for this month on the 15th in the amount of $750 dollars for all of August and half of September. I know I don’t need a reason on a month to month lease and there is no eviction amount of time on the lease. I served them with a 14 day notice on the 17th and it said any belongings left behind after the 31st would be subject to disposal. I informed them if they moved out by August 31st, or before, I would refund all advanced paid rent. Now they are trying to make me take another $250 for the remained of September rent. I don’t believe I should except it. What can I do and what should I do please?