Here is a few methods for killing tasks for a specific user in linux which may be required during account deletion or because somebody has managed to be an idiot and locked himself out with a fork bomb or some such.

The simple method is to use the utility called 'slay' which for debian / unbuntu and most other distrobutions is avilable if its not install you can install it using 'apt-get install slay'

It is very simple to use. Just running the command slay <username> and it will kill all of that users processes.

The other method to use when slay is not avilable is a combination of ps and kill. You can use the following command

kill -TERM `ps h --User nobody -o pid`

Understanding the above can other advantages as well because you can switch out the --User for --Group and kill processes by group id as well as for a specific user.