I'm a little disappointed that someone here with a CZ 50 or a CZ 70 hasn't offered some suggestions. I've never been inside either pistol, so I may be totally wrong. Take my words for what they are.
I've owned three pistols with variations on the same firing pin block, decocking mechanism, and safety lever operation, the CZ 52, the Makarov IJ-70, and the Walter PP. The firing pin block feature can fail on this design because of wear on the firing pin. All three designs rely on a firing pin block interfering with the forward motion of the firing pin unless the trigger bar raises the block out of the way. Have your gunsmith check the firing pin for wear, and check the firing pin block for correct operation. The firing pin block may be jammed in the upward position, allowing the firing pin to fly forward to strike the cartridge's primer. Or, the firing pin block may have been removed altogether. You can see the exploded view of the pistol at
http://www.gunuts.com/images/202.gif; part 007 is the firing pin and part 009 is the firing pin block. If the firing pin is worn in a way that allows the firing pin to slip past as the safety lever trips the sear, the firing pin strike the primer and fire the round.
If you're not trained in firearm repair, take the pistol to a gunsmith for repair. The CZ 70 is a simple pistol, and experienced gunsmiths who have never seen one, but have seen pistols designed with similar principles, will understand it and be able to repair it with proper parts. You should have an easy time getting it repaired to operate safely.
Out of curiosity, where are you? Your English is nearly flawless. I know from studying several languages (and mastering none) just how hard it is to learn another language. You're doing well. Keep up the good work.
Scott
