I don't know if I would use the word privileges here, because all eligible voters are given the same power, to trade or to delegate, and on the same level. Privilege suggests someone is getting something and someone is not.
In a system like Flux vote trading and delegation are features offered by the Flux Party to anyone who wants to vote on the action taken by elected candidates.
I think the question here is how can Flux effectively protect against this while maintaining a neutral non-authoritative stance? The actual coercion happens physically outside of data systems, there is no spam attacks forcing people to vote that can be traced. This disconnect between the voting technology and the physical realm is hard to overcome, which is why I rely on traditional law and policing.
You could implement a duress system, and I am not against that, but it is very complicated. When it is triggered what happens, the feedback must be that the vote was legit, but the vote has to not be counted, then you would hope there would be some reporting and investigating? Which opens up privacy and authority concerns, and again falls back on the non-Flux systems of police/legal system, there is no point calling out a coercer if nothing can legally be done about it. I get your point that you could take away their ability to participate in Flux, but that would not necessarily stop them coercing others.
Flux is bound to the law of where it is implemented, and it in turn (given enough support) could write the laws under which it is bound. In Australia in the initial stages it will be bound by existing Australian law, I don't know the law but I will make some assumptions:
You could make it a term of use that you do not softly coerce other people to vote any particular way. If evidence was presented Flux might be able to litigate as breach of contract or some other legal way if this happened, or at the least remove voting privileges. It might be practical in large scale offending but on small scale probably not.
Threatening someone with violence is probably illegal, so that would be protected outside of Flux.
Threatening someone with unemployment is probably illegal, so that would be protected outside of Flux.
Subtle coercion, such as within a family or a religious group is more difficult, how can you police that, ultimately its personal responsibility, each person has the choice to remain in that family or that religious group (and are protected by law against reprisal if they leave), again not Flux's domain to try and regulate.
Lobbyists in any form will work a lot differently under DD/Flux than what we experience now. Now they only have to influence a few politicians, under DD they have to influence entire blocks of the community, as long as law protects peoples freedom to choose, as it does, it is the persons choice if they are part of a block or not. If a majority of people support the Catholic pulpit view, brainwashed or not, then they have the right to be counted. Under the current system many Catholics might be passive, ambivalent to the more extreme policies of the church, but the church uses that passive support to force politicians to take extreme positions, under DD the church could not (in theory legally) force its members to support an extreme position, and many wouldn't resulting in it not being passed. (I think this also answers your second post about Lobbyists "hijacking" votes)
From my above note you really have to separate the types of coercion, threat of physical violence, that is illegal, wrongful dismissal or discrimination in the workplace according to political belief is (I understand) illegal. When this happens go to the police. Passive coercion to remain in a group (family/religion/etc) is a personal responsibility, you have the choice to leave that group if you feel the demands are unethical.