When deleting output formats from Web Client, output format must be deleted one by one. I expect that Web Client will be enhanced to be able to delete multiple output formats at once.
In our company, multiple output formats are published for each baseline. For example, PDF for print, PDF for proof, DITA XML for data exchange, Objects list, and so on. In addition, we create a bunch of baselines due to the revision of the document. To reduce disk space to store those outputs, we need to delete outputs for the old versions of the baseline.