You can either transfer/sell the ownership to a user (1000fg transfer fee) or delete the guild completely.
Guild FAQ:
https://forums.d2jsp.org/info.php?p=5Quote
Q: Can I transfer Guild Ownership to someone else?
A: Yes. If you are the guild owner and wish to resign and transfer your guild to someone else, I will do a transfer for you for 1000fg. Contact a mediator for more information and assistance, as this process is not automated. Please note that IRC is not included in guild transfers.
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Q: Can I delete my Guild?
A: Yes, a Guild can be deleted. This feature is shown to the Guild Owner and is under the
settings section of the Guild in the bottom right. To delete your Guild click the "Delete Guild" button. Once you have confirmed your deletion an Admin will be notified to complete the process.