lockprint-composerqgis-2.12
Is there a way to lock the print composer view extent map? This is my print composer:
If I change the scale bar in the canvas the composer change also. I try to use those options but with no success.
This is the canvas screen:
If i using "Sync composer with map" plugin 
after i chnging the scale, then the composer scale change also.
The scale will inevitably effect the extent for any given area and that will be limited by the size of your paper (so an A4 sheet @ 1:50,000 scale will be approximately a half of the possible extent of an A3 sheet). Contrariwise, setting the map extent for a given area of paper will also inevitable dictate the scale. You cannot adjust one without influencing the other (which is why you have received some negative scores for this question).
You have stated that your actual extent is not as important as scale. So to resolve your dilemma, set the scale and then, with the map selected, click the 'Move item content' button (icon is a small arrow and green box, next to the 'Select/Move item' button - a large arrow).
You can now 'slide' the content of the map without changing the position of the map on the page and thereby center you map contents on your area of interest.
If the extent of the map is still not large enough at the scale you must use, then you need to increase the size of the map and that might mean splitting your map over two or more sheets and to do that effectively you can use the Atlas generator.
With select/move object tool, resize the map area to fill the A4 space, then be sure to type the right scale (1:25000) in the map properties again. If you still have white areas, you can cut them with a software like GIMP (if you export the map as jpeg).
Best Answer
If you want to fix a map on a specific scale in the map composer, then click on the
Data defined override -> Edit ->(as you can see below) and enter the scale value that you want (only number), for example 100000.The above procedure will fix the scale, but will not fix any addtional layer that you may put on the map view. If you want freeze the map composer to show only specific layers, then click/check
Lock layers for map itemlocated under the scale.If you want freeze the extent, then copy each value of xmin, ymin, xmax, and ymax, and click
Data defined override -> Edit ->and paste each value to its corresponding box. This will fix the extent completely.