[ubernostrum]

In general, a US citizen can be detained for some time, interrogated and searched in an unpleasant fashion, have property they carry with them seized, and be subjected to (illegal, but it happens regardless) retaliatory prosecution under nuisance-type charges for being uncooperative or for being perceived to challenge the authority of the CBP employee, but cannot be denied entry to the United States.

[angry_octet]

But you can get added to the no fly list, which is effectively the same thing, as no carrier will take you. (You have to fly to e.g. Canada and cross at a land crossing.)