Can you cancel magick of your own?
yes, look up 'undo a spell' there's a few spells or chants that will undo a spell. can be a little tricky depending if it's high magic [curse] or low magic [candle spell] but it can be done. don't undo every spell you do, the universe might slap you for it. you should only cast spells when you're 100% certain. if you cast when you were really emotional and didn't think it through [like a curse] then it's understandable a spell to undo the curse. if, however, you cast a spell to see what happens, then undo it a few days later, you might receive some negative karma as a way to teach you magick isn't a game. [not saying you do that, just a warning]
Can you cancel magick of other magic users?
yes, a reversal spell or defensive magic. much like a spell to undo a previous casting, a reversal [commonly for curses] would send the spell back at the caster, resulting in the effects being reaped on the caster. the defense spells, while reverse spells would fall under them, they deal with shielding and protection so curses and other spells cast over you won't be able to effect you.