cmd: ssh -p 29418 192.168.0.1 gerritgsql
2. gerrit> deletefrom ACCOUNT_GROUP_NAMES where name=<groupname>;
3. gerrit> deletefrom ACCOUNT_GROUPS where name=<groupname>;

Project
1. access gerrit sql database
   cmd: ssh -p 29418 192.168.0.1 gerritgsql
2. gerrit> deletefrom projects where name=<projectname>;
3. gerrit> deletefrom ref_rights where project_name=<projectname>;
4. go to folder /gerrit/review_site/git, remove<project name>.git

Example for remove project:
Step 1:
[email protected]:/project/ecds/gerrit/review_site/db>ssh-p 29418 150.236.40.165 gerrit gsql
Welcome to Gerrit Code Review 2.1.6.1
(H2 1.2.134 (2010-04-23))
 
Type '\h' for help.  Type '\r' to clear thebuffer.
 
gerrit>
 
Step 2:
gerrit> deletefrom projects where name='rmproject';
UPDATE 1; 1 ms
 
Step 3:
gerrit> deletefrom ref_rightswhere project_name='rmproject';
UPDATE 1; 1 ms

Step 4:
[email protected]:/project/ecds/gerrit/review_site/git>ls
fproject.git  rmproject.git
[email protected]:/project/ecds/gerrit/review_site/git>rm-rf rmproject.git/
[email protected]:/project/ecds/gerrit/review_site/git>ls
fproject.git