Why can't you just make more clear in the beginning that this position is full-time and only full-time and that (for what ever reason) you are not able to consider part time workers for this job?
There is an information asymmetry. People want to disclose things that make them less attractive to employ as late as possible. Similarly there are things you don't tell the candidate beforehand too.
> Typically when a full time position is offered, the workload of a full person is needed
Yet when there is demand from prospective employees for something that is not offered (4 day week) but known to exist if you ask later in the interview process with some companies people will have to try if you are some company. Whether you are offering reduced time jobs or not for some positions that is available is an information you hold. You do not seem to want to hold that information back so you can make a decision depending on the candidate so you should very visibly perform send a costly signal that makes it clear reduced work hours are not possible. A costly signal would be "we are not able to consider non full-time applicants for this position".
There is an information asymmetry. People want to disclose things that make them less attractive to employ as late as possible. Similarly there are things you don't tell the candidate beforehand too.
> Typically when a full time position is offered, the workload of a full person is needed
Yet when there is demand from prospective employees for something that is not offered (4 day week) but known to exist if you ask later in the interview process with some companies people will have to try if you are some company. Whether you are offering reduced time jobs or not for some positions that is available is an information you hold. You do not seem to want to hold that information back so you can make a decision depending on the candidate so you should very visibly perform send a costly signal that makes it clear reduced work hours are not possible. A costly signal would be "we are not able to consider non full-time applicants for this position".