Can I disable interactive shell access while tunneling web traffic through SSH?

After four years this answer deserved an update. While originally I used authorized_keys myself and would probably use it still in some select cases, you can also use the central sshd_config server configuration file.

sshd_config

You can designate (for your particular use case) a group, such as proxy-only or Match individual users. In sshd_config. This is done after the global settings and revokes, repeats or refines some of the settings given in the global settings.

Note: some of the syntax/directives used in sshd_config(5) are documented in the man page for ssh_config(5). In particular make sure to read the PATTERNS section of ssh_config(5).

For a group this means your Match block would begin like this:

Match group proxy-only

You can Match the following criteria: User, Group, Host, LocalAddress, LocalPort and Address. To match several criteria simply comma-separate the criteria-pattern pairs (group proxy-only above).

Inside such a block, which is traditionally indented accordingly for brevity (but needn't to), you can then declare the settings you want to apply for the user group without having to edit every single authorized_keys file for members of that group.

The no-pty setting from authorized_keys would be mirrored by a PermitTTY no setting and command="/sbin/nologin" would become ForceCommand /sbin/nologin.

Additionally you can also set more settings to satisfy an admin's paranoia, such as chroot-ing the user into his home folder and would end up with something like this:

Match group proxy-only
    PermitTTY no
    ForceCommand /sbin/nologin
    ChrootDirectory %h
    # Optionally enable these by un-commenting the needed line
    # AllowTcpForwarding no
    # GatewayPorts yes
    # KbdInteractiveAuthentication no
    # PasswordAuthentication no
    # PubkeyAuthentication yes
    # PermitRootLogin no

(check yourself whether you need or want the commented out lines and uncomment as needed)

The %h is a token that is substituted by the user's home directory (%u would yield the user name and %% a percent sign). I've found ChrootDirectory particularly useful to confine my sftp-only users:

Match group sftp-only
    X11Forwarding no
    AllowTcpForwarding no
    ChrootDirectory %h
    ForceCommand internal-sftp
    PasswordAuthentication no

Please mind that only certain directives can be used in a Match block. Consult the man page sshd_config(5) for details (search for Match).

authorized_keys

NB: the part below this remark was my original answer. Meanwhile - but it also depends on the features of your exact sshd version - I would go for the method described above in most cases.

Yes you can, as fine-grained as you can assign public keys. In addition to nologin as recommended by ajdecon, I would suggest setting the following in front of the key entry in authorized_keys:

no-pty ssh-rsa ...

The no pty tells the server-side that no pseudo-terminal should be allocated for that key.

You can also force the execution of something like nologin for a particular key by prepending this:

command="/sbin/nologin",no-pty ssh-rsa ...

For any tunnelling-only user, change their login shell to /sbin/nologin. That way your user will be unable to access a shell on the server, but will still be able to run set up ssh tunnels from their client.

In case you are ready to give up user/pass authentication and use keys for logging in, you can specify parameters to each public key.

Notable parameters are:

command="command"

Specifies that the command is executed whenever this key is used for authentication. The command supplied by the user (if any) is ignored.

and

restrict

Enable all restrictions, i.e. disable port, agent and X11 forwarding, as well as disabling PTY allocation and execution of ~/.ssh/rc.

and finally

port-forwarding Enable port forwarding previously disabled by the restrict option.

With these you can pretty much restrict the user of that particular key pair what (s)he can do with the SSH session.

It would look like this:

restrict,port-forwarding,command="/sbin/nologin" ssh-rsa <base64-encoded key>

I know this may not be the answer you're looking for, but have you considered using OpenVPN as an alternative?

I recommend trying Tunnelier. It's a heck of a lot easier to configure/manage.