Your property management agreement continues until you terminate it in writing to the managing agents.
The reason is that the managing agents have a fiduciary duty to you to look for tenants for your property in the most time efficient way. So even if your property is now vacant - it is still binding.
However, because your property is vacant and if you don't want them to manage it anymore. You just need to let them know in writing - letter/fax/email that you wish to terminate the agreement. Some property management companies request three months notice - check with your agreement - but just tell them you may be thinking of selling - with no tenant in place, they can't say anything to you and you won't be up for any fees. I've never heard of management companies to charge for release this way - but if they pose a problem, let me know.
Hope this helps.
