How to restrict aclass not to be accessible for any class to consume, by creating it's object?

Have a private constructor in that class. So that it can't be instantiated. Also refer singleton patterns

Albin is right. However, I need to make this answer more accurate.

You should makes sure that all defined constructor's of the class are private (which is the default if there is no access modifier), that is, none of them should be internal or public.

If there are absolutely no constructors, a class still can be instantiated, because an implicit (hidden) parameter-less constructor will be provided (only if there is no other constructors). In this case, if such parameter-less constructor is explicitly provided and private, the class cannot be instantiated; if this is the only purpose for creating such constructor, its body can be empty. (Thanks to Robert Rohde who helped me to fix my mistake in first version of this answer. Sorry for confusion.)

This leaves for the possibility of a back door: such class might have an internal public static function which returns an instance of this class. As such method has access to private constructors, this could be the remaining way to create an instance. You decide what to do with this possibility.

Finally, you can remove the possibility of instantiation by simply declaring the whole class as abstract, even in the case when not members are abstract. The only purpose of such class will be to serve as a base class.

—SA