Are there ethical guidelines prohibiting penalties against potential subjects who withdraw from a research study before providing 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 a participant recruitment system, whereby every time a student signs up for a research study, they occupy a 'seat' out of a limited number of seats available. (Research credits are distributed fairly among researchers to ensure that everyone is able to recruit in proportion to their needs). Some students do not have the courtesy to cancel their seat if they decide they no longer wish to participate, which is a problem because there are deadlines for recruitment and if students do not cancel before the deadline, they are preventing other would be subjects from participating. As a result, some researchers may be unable to meet their recruitment goals. Although this scenario applies to online studies, there are certainly other scenarios where careless behavior results in major inconveniences to the recruiter, for example no-shows unaccompanied by any excuse.

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 could involve "unspecified" penalties? (In this case, there would be no penalties but the warning would serve as a deterrent.)