I have found solution for this – you just have to go to setting of forum you created and choose “Closed”-state for forum. Then no one can write anything on forum particular forum 🙂 Thanks anyway for help!
Ok, I was using originally “Members”-called plugin instead of WPFront User Role Editor. Now tried to use WPfront, but when I’m trying to uncheck those and save it, it doesn’t uncheck them. Maybe issue is in “Override Edit Permissions” ? I check it and hit “Save” button and it says “If you have a caching plugin, clear the cache for the new settings to take effect.” Where can I clear it and what exactly?
Challenge is that since forum is private you cannot see it.
Here is the picture of my forum structure: 
That is the my Main page for forum. From this page I need to disable topic posting ability, so users cannot create any posts like above the posting box: Test, Test 2, Test 3 and so on…
So, the only thing there should be left are all the main topics which I created (the most upper ones). Then, when you go to one of those predefined topics, there whoever (participant …) user would be, he can create own topics within this main topic. So, topic posting would be working everywhere, BUT Main forum page.
I hope this will clarify more 🙂
@jessy-marco: I have tried your solution in different ways (I need ability for users to be able to post their topics as subtopics of main topics). Now I can disable completely, as you said, capability for creating topics but I need to disable it ONLY at MAIN page. Any advise for that?
Ok, thanks for clarifying things, but I’m not familiar with modifying php-code, so I still would need help in this problem :/
“You want this: Do not allow users to publish new topics, just replies on your topics. ”
Almost, I want to restrict users from creating their topics on MAIN page of forum because I have already created kind of “locked” main topics, eg:
I have created forums:
Main topic 1
Main topic 2
Main topic 3
Users can then go to these topics above and CREATE their own subtopics there and only there, that’s the idea. So they are not allowed to create topics right on MAIN page of forum, but otherwise they can. This is kind hard to explain :/
@Robin W: I tried to disable lines 42 and 43 from form-topic.php but not sure, where in my theme folder I should upload this modified file.
I’m sorry, maybe I wasn’t clear enough with my problem but basically this (picture above) what I need to remove from all the users (keymaster, participants) from the start page of forum where all main topics are located: 
Why I want this is because I want to prevent users from creating their topics on the start page of forum. Of course, topic posting would then work in main topics which I created already. Thanks!
So, I have managed to hide “post-form” by editing “bbpress.css”-file
Added line “display: none;”
#bbpress-forums fieldset.bbp-form {
display: none;
clear: left;
But this hides “post-form” completely so I’m not able to, for example, reply any topics there are in forums. Anyone?
