Modifying Actions File
If you're careful, you can modify an actions file. However, you must be careful to keep the correct XML syntax, otherwise you'll see an "Invalid Actions File" error when you attempt to load it. It may be prudent to make a copy before making any changes.
To remove an entire action, simply remove those lines describing the action. For example, to remove the dirt dropping action below, remove the yellow-highlighted lines:
...
<actions>
<dirt>
<dirtPerTile>40</dirtPerTile>
<maxDirtSlope>40</maxDirtSlope>
<maxDiagSlope>56</maxDiagSlope>
<maxDirtHeight>3600</maxDirtHeight>
<waterHeight>500</waterHeight>
<cliffRatio>2.5</cliffRatio>
<landslides>false</landslides>
<biomeSeed>852576059348</biomeSeed>
</dirt>
<biome>
<tileType>TILE_SAND</tileType>
<seedCount>200</seedCount>
<biomeSize>100</biomeSize>
<biomeDensity>1</biomeDensity>
<maxSlope>30</maxSlope>
<minHeight>50</minHeight>
...
To change a value, just replace the existing value with the one you want:
...
<biome>
<tileType>TILE_SAND</tileType>
<seedCount>200</seedCount>
<biomeSize>100</biomeSize>
...
might become:
...
<biome>
<tileType>TILE_SAND</tileType>
<seedCount>300</seedCount>
<biomeSize>100</biomeSize>
...