Hello. I'd like to allow specific GPU(GPU in AMD 7950x), not 3080, to compute.
Is there any settings? Or mb task filter with tasks for AMD GPUs?

With Notepad, make a file called cc_config.xml in your BOINC Data directory (if it not already exists) and add into it:

<cc_config>
<log_flags>
</log_flags>
<options>
<<ignore_cuda_dev>1</ignore_cuda_dev>
</options>
</cc_config>

Save it as cc_config.xml, making sure it doesn't get an added .txt (which Windows likes to do), and make sure it's set to UTF-8 encoding.
After that, exit BOINC completely and restart it.

The system has 1 AMD GPU and 1 NVIDIA GPU, so shouldn't it be
<ignore_nvidia_dev>0</ignore_nvidia_dev>

According to the manual (https://boinc.berkeley.edu/wiki/Client_configuration) it should be <ignore_nvidia_dev> instead of <ignore_cuda_dev>.
The latter is deprecated but still works according to the source code.

Please remove the typo at the beginning of line 5.
'<<ignore...' should be ''<ignore...''.

Well, I can't remove typo's anymore than you can. But you're right, it should've been nvidia instead of cuda.

<cc_config>
<log_flags>
</log_flags>
<options>
<ignore_nvidia_dev>1</ignore_nvidia_dev>
</options>
</cc_config>