Hi, try with PBS-triton x-100

There are reagents that will remove the antibodies of a first western blot to allow you to reprobe the membrane for a second protein of interest (Western blots stripping buffers). But there is no reagent that will remove the blocking agent from the membrane without also removing your protein of interest. If you have blocked the membrane with 5% milk and you are looking for a phosphoprotein, it is likely that you will have a lot of background, as casein, a main protein in the milk, is phosphorylated. It will depend on the affinity of your primary antibody and whether it is monoclonal or polyclonal. If it is a monoclonal antibody recognizing a nonphosphorylated domain you should not have background problems. I would continue with the experiment and evaluate the results. If they are bad, then is recommended to start a new western blot blocking entirely with BSA