To limit user process just add user name or group name or all users to /etc/security/limits.conf file and impose process limitations.
For example:
# To impose a process limit of 500, on "cse" group, add the following line to the file.
@cse hard nproc 500
# To impose a process limit of 100, on user "galvin", add the following line to the file.
galvin hard nproc 100
For more information see the man page, command is "man limits.conf"