Can I use my hand to kill mosquitoes when they disturb me in Salaat? Because they keep on biting, breaking the concentration in salaat.
From Fiqh-us-Sunnah - 2.83:
If killing these would only require a small action on the part of the person in salah, then there is no harm in doing it.
Abu Hurairah reported that the Prophet (PBUH) said:
"Kill the snake and the scorpion during the salah."
This is related by Ahmad, at-Tirmidhi, Abu Dawud, an-Nasa'i, and Ibn Majah. The hadith is hasan sahih.
It is permissible to exterminate bugs and insects which are harmful to one's life or belongings. Source
In principle, it is permissible to exterminate bugs and insects which are harmful to one’s life or belongings such as pests. Bugs and insects which are not harmful should not be killed. Harmless insects should be gently moved away without killing them.
When exterminating an insect or bug, one should not oppress the creature. The creature should be exterminated quickly without giving it pain. Maximum effort should be made to minimise suffering and pain. It is prohibited to give undue pain to the creation of Allah. 
It is severely disliked and prohibited to burn or drown any creation of Allah. Only in the extreme case where there is no other strategy to safeguard oneself and property from the harms of a creature can one resort to burning or drowning.
You can shove away or kill mosquitoes if you find it's harmful. It is advisable to take necessary precautions to avoid such circumstances. Use mosquito coils/aerosol/insect killer before starting your prayer.