To Remove Effects on a certain area, you can't Mark In and Out, you have to enter Segment Mode and do it. Easiest way is to ALT + "lasso" (left to right) over the area you want to remove effects from which automatically puts you in Segment Mode and selects those clips), and hit the Remove effect button (note: this will only Remove the top most layer of effects, so if you have nested effects, you may well need to do this a number of times to be safe).
Now, to remove Dissolves is a little trickier.
Weirdly, you have to be in Effects Mode. Select a Transition, then "lasso" (left to right) over the area BUT MAKE SURE THAT YOU START LASSOING ABOVE THE TIMELINE, NOT ON IT. This selects all the transitions for tracks selected. And hit the Remove effect button.