HOMER doesn't have a checkbox to disallow wind-only operation, but if you set the operating reserve as a percent of wind power to 100%, you get the same effect.  Say you do that and you also set the operating reserve as a function of hourly load to 30%.  Then in some hour if the load is 50 kW and the turbines are producing 80 kW, the required operating reserve will be 30% of 50 kW plus 100% of 80 kW, which is 95 kW.  The wind turbine supplies 80 kW of that, so the diesel(s) have to provide 15 kW of operating reserve, just as they would if the wind turbines weren't operating at all.  So whenever the primary load is greater than zero, the diesels have to operate.  Hence wind-only operation is not allowed.