How can I drop all indexes in a SQL database with one command?

You’re very close.

declare @qry nvarchar(max);
select @qry = 
(SELECT  'DROP INDEX ' + quotename(ix.name) + ' ON ' + quotename(object_schema_name(object_id)) + '.' + quotename(OBJECT_NAME(object_id)) + '; '
FROM  sys.indexes ix
WHERE   ix.Name IS NOT null and ix.Name like '%prefix_%'
for xml path(''));
exec sp_executesql @qry