Short answer: don't treat them different from other philosophers. Look at the ideas, not at the people who proposed them.
You could have a look at the recent question "Cherry Picking Fallacy (In a Social Media Debate)" and my answer to it. It is about an argument between two people concerning a writing by a Christian philosopher. One of the two (the 'dialogue partner' below) claims that, since the philosopher is a Christian, he is biased and his work is worthless / should not be considered.
Propose your dialogue partner the following:
- We define some unbiased machine M that outputs some theory T. This T is randomised.
- Now suppose T is exactly the theory as described by the author you're discussing.
- Since T was proposed by a christian, his work is deemed "worthless".
- However, following the underlying assumption as I described before, since T was proposed by an unbiased author, his work needs to have some attribute to show that it's worthless.
In other words, ask your partner to precisely define his notion of worthlessness, and then ask him to show that the work you're discussing is worthless in a way irrelevant of who wrote it (to avoid the ad hominem).
If you would treat racist philosophers different from other philosophers, you are, as the dialogue partner in this story, doing Cherry Picking as well: you're considering only that which you like, which matches your own beliefs, et cetera.
Instead, you should consider everything equally. If you then have some argument against philosophical racism, that's okay. But it should be based on the works you have read, not on the word 'racism'.
Another example, say I claim:
- It is moral to be nice to my neighbours.
- It is moral to kill people from outside my country (I do not think so - this is an example)
Then, would you say all my claims are worthless because of the second one? That seems unfair, and most people would agree with the first one. Instead, you should consider all claims separately (but, with respect to the rest of the ideas to which they relate), because even racist philosophers can be right, sometimes.