How can I comment out a block of text in texmaker?

To comment several lines at once in TeXMaker, select the lines and click Ctrl+T. To uncomment, click Ctrl+U.

Edit

To follow up on Torbjørn's comment, you can see the mostly used keyboard shortcuts under menu

Edit > Comment

You will see all of the default keyboard shortcuts and customize your keyboard shortcuts under menu

Options > Configure Texmaker > Shortcuts

One of the shortcuts I find helpful is Ctrl+Space which toggles between pdf viewer and editor, especially when I use a lot of applications and workspaces at once.

In LaTex or PC Tex: Select the text you want to block or turn into a comment and use Ctrl Shift % Hold all three keys at once.