I checked the USATF Rules Book (because I hate the WA book) and it says the following:
Rule 143.5c
"(c) No athlete shall be permitted to participate in any competition without the appropriate bib, except as indicated in these rules. Bibs are to be worn as issued (cutting or folding numbers is not permitted). The identifying information on the bib need not be numeric. It shall be of sufficient font type, size and contrast for clear visibility. Not wearing an assigned bib may be grounds for disqualification under Rule 145.2."
Rule 136.4
"The Clerk of Course, and Assistants, shall ensure that competitors are wearing clothing, shoes, and spikes which are in accordance with USATF Rules. The Clerk shall verify that bibs correspond to those on the start list and are worn correctly. The Clerk shall also be responsible for distribution and proper placement of additional identification items used to aid in announcing and judging."
Rule 145.2
"Any competitor who shall refuse to obey the directions of the Referee or other proper official, or who shall conduct himself/herself in an unsporting manner, or who is offensive by action or language to the officials, spectators, or competitors at any competition, or under Rule 162.17, shall render that competitor liable to disqualification by the Referee from participation in all further events or rounds of events, including individual events of a Combined Event, simultaneous events and relays, at the meet. If such disqualification occurs, any performance accomplished up to that time in the same event session shall not be considered valid. Performances accomplished in any previous event session, such as a qualifying round or previous events of a combined event, shall stand. If the Referee thinks the offense worthy of additional action, he/she shall promptly make detailed statement of the offense to the appropriate National or Association Officer."