For Water Base Emulsion:
What actually works best to blockout a screen for WB ink is the emulsion itself. You can just spread a little on the area of the screen that needs to be blocked out and then let it dry, put it in the sun or on the exposure unit and it will work great!
For Plastsiol and Solvent Emulsions:
There are Block out pens and Redcoat. You can spread them in the areas you need to block out and set them to dry in front of a fan. If you are blocking out pinholes you can do this on press as they as fast drying coats.
Scotch tape works well for pinholes on the t-shirt side of the screen.