How can i remove a rest at the end of a measure

You seem to have added a half beat to the measure. The blue + tells you that happened. My guess is that you added a note using ctrl+shift+notename and inserted a note. To get rid of the extra rest select it and press ctrl+delete. Note: this is the timewise delete in the right click menu and the tools menu. When the extra note is gone, the blue plus will disappear.

The blue + indicates an irregular measure, i.e. a measure that has more beats to it than the time signuature tells. A blue - likewise indicates a measure that has less beats, like in a pickup measure