Yes, but only if you have to understand what you are doing.
In ST4, each sound consists of an "instrument-definition" file (with a .st4i file extension) and one (or more?) sample files (with a .pak file extension).
All you need to do is remove the files you don't want. It would be best to back them up somewhere, so you can bring them back if needed. The problem is finding out which ones to remove. There is no published information.
The best thing to do is
contact Support, via here. If Searching the FAQs does not help there is a
red button link to the Technical Support form, for some one to one help. Be s specific as you can about which sounds you want to remove. Response time aims to be within 2 working days (US (East Coast) Business hours, generally), but in the current circumstances please allow some more time.