There is a write only mode in rsync version 2.6.3 or later (for implementation details see commit 7a92ded39a). It behaves as you described in your question.

Configuration in rsyncd.conf:

write only

This parameter determines whether clients will be able to download files or not. If lqwrite onlyrq is true then any attempted downloads will fail. If lqwrite onlyrq is false then downloads will be possible if file permissions on the daemon side allow them. The default is for this parameter to be disabled.

[module]
path = /some/path
read only = no
write only = yes
...

If you try to receive files from a write only repo you will get an error:

$ rsync rsync_host::module .
ERROR: module is write only
rsync error: syntax or usage error (code 1) at main.c(804) [sender=3.1.2]

There is a write only mode in rsync version 2.6.3 or later (for implementation details see commit 7a92ded39a). It behaves as you described in your question.

Configuration in rsyncd.conf:

write only

This parameter determines whether clients will be able to download files or not. If "write only" is true then any attempted downloads will fail. If "write only" is false then downloads will be possible if file permissions on the daemon side allow them. The default is for this parameter to be disabled.

[module]
path = /some/path
read only = no
write only = yes
...

If you try to receive files from a write only repo you will get an error:

$ rsync rsync_host::module .
ERROR: module is write only
rsync error: syntax or usage error (code 1) at main.c(804) [sender=3.1.2]