This plugin disables chat spamming in a different way that usual.
Others limit the delay between each chat message.
This plugin disallows the same message to be said by the same person more than X times.
If the same message is said more than X times by one person, then that person cannot say that message anymore.

antispam_enabled <0|1>

• Turns plugin on/off
• - 0 = Disabled
• - 1 = Enabled
• Default: 1

antispam_samecount <positive integer>

• The maximum number of times a player can say the same message before it is blocked.
• Default: 3

antispam_minlen <positive integer>

• The minimum length of a message for it to be considered spam.
• This is intended to rule out commonly used acronyms such as "lol", "omg", etc.
• Note: Spaces before and after the message are not counted for.
• Default: 6

antispam_warndelay <seconds>

• Delay between "Stop spamming messages!" warning message is shown to player.
• Default: 1.5

antispam_slash <0|1|2>

• If your server has slash commands for chat, then you should use 1 or 2.
• - 0 = Disabled
• - 1 = Slash for normal chat
• - 2 = Slash for normal and team chat
• Default: 0

Attached to this post, there is a file called antispam.ini.
Open that file and there are instructions on how to use it.
When you are finished, place it in your server's addons/amxmodx/configs folder.

- AMX Mod X 1.8.1 or higher

• Version 0.6.1
  - Minor fix from v0.6 update

• Version 0.6
  - Fixed antispam.ini only working for slash commands

• Version 0.5
  - Fixed error when reading file.

• Version 0.4
  - Added compatability with normal chat commands such as /me, timeleft, etc.

• Version 0.3
  - Minor bug fix.

• Version 0.2
  - Fixed few typos.
  - Added antispam_slash cvar.
  - Added better compatability with adminchat.amxx

• Version 0.1
  - Initial Release

• joaquimandrade
  - Helped me with bits and pieces of the plugin.

• Version 0.3
  - Minor bug fix.

• Version 0.2
  - Fixed few typos.
  - Added antispam_slash cvar.
  - Added better compatability with adminchat.amxx