OpAmp: Calculating A - MAX(B, 2V)
Here is one easy way, but probably not the best way given your actual situation!

Since you say you have all sorts of supply rails and references available, this way requires +/- supplies and +/- references.







simulate this circuit &ndash; Schematic created using CircuitLab

R2/-10V reference determines where the input meets the clamped voltage
R5/R6/+10V reference determines the clamp voltage 
OA2 and OA4 are just buffers.