Q. How do I remove a property, such as a user defined property, which cannot be accessed through the Propeties Editor?

A. Set it to nothing, for example:

OM(SingleWindowApp)-> a<myProp=6>; 
OM(SingleWindowApp)-> $print a int a<myProp=6> = 3; 
OM(SingleWindowApp)-> a<myProp= >; 
OM(SingleWindowApp)-> $print a int a = 3;

Be sure to include the space after the equal sign.