To stop certain html tags from appearing in message posts, you will want to change settings in the HTML Filter following these steps:
1) In the admin console click on the "Spaces" option on the top bar
2) Click the "Settings" tab below the bar
3) In the menu on the left click on the "Filters and Macros" option
4) In the section labeled "Pre Processing Filters", there is a filter called "HTML". Click on the "Settings" link to the right.
5) There is an option in the settings screen called "Disallowed HTML tag". Append the html tags you want to have filtered out to the list in the text box below the label. So, for the example tags you gave, you would append "table,tbody,tr,td" to the list.