There are several possibilites. You should configure the file /etc/systemd/journald.conf. Two relevant options (from the man page) are:
Configures the rate limiting that is applied to all messages generated on the system. If, in the time interval
defined by RateLimitInterval=, more messages than specified in RateLimitBurst= are logged by a service, all
further messages within the interval are dropped until the interval is over. A message about the number of
dropped messages is generated. This rate limiting is applied per-service, so that two services which log do not
interfere with each other's limits. Defaults to 1000 messages in 30s. The time specification for
RateLimitInterval= may be specified in the following units: "s", "min", "h", "ms", "us". To turn off any kind of
rate limiting, set either value to 0.
Controls where to store journal data. One of "volatile", "persistent", "auto" and "none". If "volatile", journal
log data will be stored only in memory, i.e. below the /run/log/journal hierarchy (which is created if needed).
If "persistent", data will be stored preferably on disk, i.e. below the /var/log/journal hierarchy (which is
created if needed), with a fallback to /run/log/journal (which is created if needed), during early boot and if
the disk is not writable. "auto" is similar to "persistent" but the directory /var/log/journal is not created if
needed, so that its existence controls where log data goes. "none" turns off all storage, all log data received
will be dropped. Forwarding to other targets, such as the console, the kernel log buffer or a syslog daemon will
still work however. Defaults to "auto".
Thus you can turn off all syslogs by means of Storage=none, but this seems excessive and unwise. Limiting the rate (the first option= is much wiser; the default is 1000 messages per 30s, which is way above what you wish, and explains why you are having so much output, unchecked.
Or, you may use one of these options,
SystemMaxUse=, SystemKeepFree=, SystemMaxFileSize=, RuntimeMaxUse=, RuntimeKeepFree=, RuntimeMaxFileSize=
to control the size of journal files stored. You can pair these options with one of these,
The maximum time to store entries in a single journal file before rotating to the next one
The maximum time to store journal entries.
to retain all syslog messages, but for a shorter time.
Lastly, you may wish to use:
MaxLevelStore=, MaxLevelSyslog=, MaxLevelKMsg=, MaxLevelConsole=, MaxLevelWall=
Controls the maximum log level of messages that are stored on disk, forwarded to syslog, kmsg, the console or
wall (if that is enabled, see above). As argument, takes one of "emerg", "alert", "crit", "err", "warning",
"notice", "info", "debug" or integer values in the range of 0..7 (corresponding to the same levels). Messages
equal or below the log level specified are stored/forwarded, messages above are dropped. Defaults to "debug" for
MaxLevelStore= and MaxLevelSyslog=, to ensure that the all messages are written to disk and forwarded to syslog.
Defaults to "notice" for MaxLevelKMsg=, "info" for MaxLevelConsole= and "emerg" for MaxLevelWall=.
to forgo less important messages, and retain only critical ones.