Skimlinks respects all pre-existing affiliate links on a page, and we do not overwrite any links that already have affiliate tracking. Our script will only re-write normal merchant links without affiliate tags, where we are part of the affiliate program.
We can blacklist your site from specific merchant programs, but this will mean you will not earn commissions from that merchant anywhere on your site. If this is something you would like, please submit a ticket here.
Alternatively, if you just want to exclude a specific merchant link, block of text or page rather than all links you have to a Merchant, please see our documentation here.