Well yes - they're free to tell them to leave (speech), and free to compel them (property). Interestingly some theorists consider freedom of speech a special case of property rights.
In the case of a platform like Twitter, blocking certain types of speech is also a meaningful expression in itself. Telling someone to leave is a form of speech in itself. But telling (for example) many environmentalists to leave would also express a view on environmentalism because of the effect it would have on the overall conversion. Shaping the discourse on the platform is similar to a media outlet shaping the discourse it drives with selective reporting.
