How to destroy Parameterized object with watchers and depends decorator

I have two parameterized classes: first is with settings, second is uses ClassSelector(class_=first) and has additional data.
And I dynamically add and remove Second class instances. But how to delete Second classes entirely, because callbacks is activated even after deletion.

import param as pm

class A(pm.Parameterized):
    a1 = pm.Number(10)
    a2 = pm.Number(100)

class B(pm.Parameterized):
    b = pm.ClassSelector(class_=A)
    def __init__(self, **params):
    @pm.depends("b.a1", watch=True)
    def callback(self):
        print(f"a1={self.b.a1}, a2 = {self.b.a2}")

a = A()
b = B(b=a)
c = B(b=a)

a.a1 = 12

del b

a.a1 = 13