There are a number of warnings available in Delphi XE2 that are not very well documented.  While you can control them in the Project options dialog, and you can turn them on using {$WARN} directives or in command line compiler options, the documentation for the warning identifiers is currently pretty piecemeal, and there is no clear link between the project options, warning directive identifiers, and numeric identifiers.

The {$WARN} compiler directive takes effect only in the unit it is in.  It overrides project settings and command line compiler flags.  To turn a warning on or off for a block of code (although you’ll have to work hard to convince me that this is an acceptable solution 99% of the time):

{$WARN SYMBOL_DEPRECATED ON}
{$WARN SYMBOL_DEPRECATED OFF}

To restore the warning to its previous setting:

{$WARN SYMBOL_DEPRECATED DEFAULT}

And finally, you can convert a warning into an error:

{$WARN SYMBOL_DEPRECATED ERROR}

To change the setting in your project, go to Project|Options, Delphi Compiler, Hints and Warnings, and expand the Output warnings setting:

Delphi’s project options dialog – hints and warnings

To change the setting in the command line compiler, use the -W flag, for example:

dcc32 -W+SYMBOL_DEPRECATED myproject.dpr
dcc32 -W-SYMBOL_DEPRECATED myproject.dpr

Or to turn the warning into an error:

dcc32 -W^SYMBOL_DEPRECATED myproject.dpr

Note, if you type this from the Windows command prompt, you will have to repeat the ^ symbol, as ^ is an escape symbol:

dcc32 -W^^SYMBOL_DEPRECATED myproject.dpr

These are the hints and warning identifiers I am aware, along with what I understand are the default settings and numeric identifiers, and links to the relevant help topics.

The $MESSAGE directive allows generation of H1054, W1054, E1054 and F1054 messages.  User-defined hint and warning messages can be turned off with {$WARN MESSAGE_DIRECTIVE OFF} but logically enough, user-defined error and fatal messages can not be disabled.  For example, add the following line to MyUnit.pas:

{$MESSAGE WARN 'This is a user warning'}

Which will gives a compiler warning similar to the following:

[DCC Warning] MyUnit.pas(25): W1054 This is a user warning

Finally, I’m not going to discuss the $WARNINGS directive, except to say that I don’t think it’s ever necessary or sensible to turn it off.  If you must turn a warning off for a specific section of code, turn just that warning off as discussed above.  Using {$WARNINGS OFF} is just as unhelpful as an empty except block.