[Erwin]

Do you mean add a column in the user table?

Run this query:

ALTER TABLE user ADD spamcount

That would add a column called "spamcount".

You can also use a custom user profile field for this, which is easier to handle. Just go to the Admin CP, and add a custom user profile field. Make it not editable and not viewable, so only mods can see it in the CP. You can then add to it for spamming.

Alternatively, use g-force's excellent warning hack, which does this all for you and much more.