I haven't tried it, but would you be able to try uninstalling and re-installing IPAM possibly do it? Otherwise, maybe look through the IPAM_xxxxxx tables in the database and see which ones contain all relevant data and start truncating them. I forget the exact table names but I know similar to IPAM_Nodes and IPAM_Groups carry the individual IP and subnet data respectively, other tables probably carry more info you may need to truncate.
Truncate at your own risk! Backup that database before trying it. If possible, backup your database and then try it out on a test VM so you can see if it works or not.