Are there ethical guidelines prohibiting penalties against potential research subjects who have not yet provided consent? Are there any ethical guidelines that would prohibit the use of penalties against individuals who sign up for a research study but have not yet provided consent and are therefore not yet officially subjects?

Please provide references. 

Background

In the psychology department, we have an online recruitment system, whereby every time a student signs up for a research study for extra (course) credit they occupy a 'seat' out of a limited number of seats. This limitation exists due to the fact that research credits are distributed fairly among researchers to ensure that everyone is able to recruit in proportion to their needs. 

Online studies pose a particular challenge for the recruiter because the participant is afforded the opportunity to complete the study by the end of the semester and there is no guarantee that they will follow through. The problem is that many students do not have the courtesy to cancel before the deadline if they decide they no longer wish to participate, and in doing so they are preventing other students from participating. Not only is this behavior not fair to other students, but it also means a loss of potential subjects for the recruiter. Although this scenario applies to online studies, there are certainly other scenarios where careless behavior results in major inconveniences to the recruiter, for example people not showing up for appointments (i.e., in-person studies, experiments). 

If those signed up for a study have not yet provided consent (i.e. are not yet officially subjects), what are they protected against? Specifically,

1) Is it admissible to issue penalties, such as, for instance, a loss of research credits?

2) Is it admissible to communicate a general warning to all individuals using the system that a failure to cancel their seat or to show up for appointments could involve unspecified penalties? (In this case, there would be no penalties but the warning would serve as a deterrent.)
From the US Code of Federal Regulations (45 CFR 46), in the section on seeking consent:


  An investigator shall seek such consent only under circumstances that provide the prospective subject or the representative sufficient opportunity to consider whether or not to participate and that minimize the possibility of coercion or undue influence.


Note the use of the phrase "prospective subject" - this clause refers to the investigator's conduct towards potential subjects who have not yet given consent. Presumably a penalty or threat of a penalty would constitute "coercion or undue influence."

Specifically regarding research participation for credit, the APA Code of Conduct Section 8.04 (a different section of which was mentioned by Stephan Kolassa in his answer) says


  (a) When psychologists conduct research with clients/patients, students or subordinates as participants, psychologists take steps to protect the prospective participants from adverse consequences of declining or withdrawing from participation.
  
  (b) When research participation is a course requirement or an opportunity for extra credit, the prospective participant is given the choice of equitable alternative activities.