If I am not interested , I will always refer them  to someone who can help them.
if I am interested but am struggling to fit them in , I may tell them that they  will have to wait a couple of months .  ( if its an add on )
I try to accomadate all window cleaning inquiries and charge accordingly.  
the only time I block a number is if I have dropped a customer for being a bad payer, or a habitual skipper of cleans.
I tend to do any add ons on a saturday morning , as my son will help me . its quality time spent together , and cash to help him whilst hes in college. 
generally  this is two saturday mornings a month. 
I like to build and refine , or build and price increase ...