They should not give reason to them for terrorism and cut their support. However, countries are supporting terrrorism in order to obtain their purpose in another country. That is befrore cutting their support, it is very hard to stop terrorism. Nothing we can do until countries come together and find a solution. This could stop their financial support.
Another thing is community support. They have an aim and they have some believers convincing other people to join. Goverments should build a political strategy in order to stop their support. If government provides better conditions to terrorism supporters they will stand by the government and stop supporting terrorism.
The last one open a suitable way to them to join community and leave terrorism. If they choose to leave with their own will they could get very low punishment as a reward. Of course they should not even contact them again.
These could be some options for solving this issue.