Yes, you can just send a written notice of default and to terminate his services in writing. There is not a 30 day notice period required unless the written property management agreement requires it.
The agreement should outline the fees and compensation structure for the property management services. This may include a percentage of the monthly rent, flat fees, leasing fees, maintenance charges, and any additional expenses that the property owner might be responsible for.
In addition to the length of the agreement, a property management contract covers everything the property manager will do for the owner and fees for the service. The contract also stipulates which party is responsible for the functions of the daily operations of the property.
A property manager becomes an authorized representative when the property owner & manager execute a legally binding property management agreement, a legal contract that details a manager's responsibilities and the owner's expectations regarding the property's managerial tasks.