new word[][] = 
{
"word1",
"word2",
"word3"
}

public OnPlayerText(playerid,text[])
{
//..........code there, without return 1;
for (new i;i!=20;i++)
{
if (strfind(text,word[i]) != -1)
return 0; //this will disable sennding that in chat;
}
return 1;
}

stock str_replace(needle[], replace[], haystack[])
{
    new index = strfind(haystack, needle, true);

    if(index == -1) return false;
    
    strdel(haystack, index, index + strlen(needle));
    
    strins(haystack, replace, index, strlen(haystack));
    
    return true;
}

public OnPlayerText(playerid, text[])
{
    str_replace("tree", "leaves", text);
    return 1;
}