For everyone still coming to this post, as me, after many years, since we’re beginners: a straightforward way, if I understood correctly, is to define your rv as you would normally do, and exploit the observed=<value> argument inside the definition of the rv.
E.g. Fix the parameter “A” to a value of 0 and don’t calibrate it
A = pm.Normal('A', 0, 1, observed=0)
This is particularly useful if you, as me, have a model with many parameters and want to perform multiple calibrations by fixing different parameters every time.