If I remove lists from the Audience section of a campaign, will associated prospects be removed from the campaign?

No, any prospect that is already a member of a DemandCenter campaign will remain in that campaign indefinitely. If the list they were in is removed from the Target Audience option of the campaign's Start Action, they will still remain in the campaign to proceed through the campaign flow(s) as designed. Prospects will not restart any flows, repeat any actions, or in any way deviate from the original intended behavior of the campaign.