How can I isolate an rf signal?
There are a couple of things that you can do.

1) insert a low-power buffer amp between the output of the signal generator and the power amp.  Quite frankly, you are going to need multiple amplifier stages anyway.  So make the first amp stage function as a buffer.

Have a look at Mini Circuits for really easy-to-use single-chip solutions.

2) Add a couple of big-ass diodes back-to-back across the output of the signal generator.  3.2 mW is about 0.4V P-P, so a pair of silicon diodes connected back-to-back from the signal pin to ground should protect the generator should something bad come back towards it.

Be sure to choose diodes that have low capacitance.  I know that it can be hard to find the combination of low capacitance and high-current capability but I know they exist.  Unfortunately, I don't recall where I saw them.