Plugin Dev – stop plugin from using theme CSS

Not really enough information for a fool-proof answer but for starters, make the plugin CSS plugin specific. So instead of writing for example:

ul { color: red; }
li { list-style: square; }

Write it more like:

#your-plugin ul { color: red; }
#your-plugin li { list-style: square; }

You might need to be even more specific than that though.